








Eg ON ts ee ee Ae ST ana a ote oo 
at a ae fe re Fate 28 Beings >See ai 
Big a re vans ias 
et AOE eee 
sea ee ee 





= wes mi 
op aS se eS aT SRT ee pom ne 
ois aan = i ie 
~ See ata ene - 
et er 2 poe axe 
f Tp) ae SS See 














an ee 


= 
ee 
pte 





WahA 
= 
" 
prette POSE Ca Wet 
> wat. we tatet Aes ate Cee 


ANT 
My . 
4 


i} 


HRMS 
Abr eats 


fist why a 


ee ee 
se Se ae na ae a ge 
es = | = 





. 
= 


ry 


ives yi 
HANDS FAIL 
a pecapinny sea r Seer rer arte it er, 
See : : ms , Eo acre Sa me by 
YAM REAR REGEN MA TES BRO ARAL TE hPa NS } 


TRAP ON ASCH WAS AAL 





( Be 
« 
‘= 
i # 
fe Le 
oT iz 
=< AS 
= oe 
es 1s 
as td 5 
~~ 
ee . ~ 
Fe ix : .z 
< 
Ne ee y + 
= 4 ~ 
—s = See: —— Sn = Ol re ee P| 
eo ed = 2 TE ng ney PO Fe Tg Oe See Pe a 
. < SNe a xp se 38 a ee ee ae > at — ea 
Pas ” * = “ So ee _—— (a ees ee ng ORE a a ae agreeing ene ee ee a = eta ee ge a 
a ae —— I ee Oe ae enon ea et I wl ee te See eagle a ad 
ae j . pinata A a ao ee a - ~ re ce ape ee Se ne na ae Fa Gp hans en ne rn ag ee ee a ae - 
Pees Sy Ee atin” Is - pte Bee “ “ 2 om nat A i Soca OT EY oa Pn af ee ig ae emer, on eaten of Se re SS ee Sa Te Pe — ee ge a apie 3 
OS ee . Ss ang hE Bl ng aS aah eg Oe ea aE, fe ee a GE ag ‘ os 5 gta a - Oe OS TE ag A peta aN, Np rte aia ee apermPue nie ere neat ee ey 
“~ 2 oe ee i a ng a AED pl nag IAI BO ae z ~ eae ee PO pe ae SO pee a ee en, pate nnn ng 
PO ee cima Se OI IPT Ni ag og BET ie: a i Se oe pee ig nag Oe nea nn Sa Poa ge a PM pe. 
SS 3 SS eee : SIF 
na Pe at oe eo — << = ~ ’ a > —_ ‘-= = - OS oF a ee at eal ag = . 
we, Oe, 2p age A ta Sr Se ae gag . ~ - el P = w ee ae oP ne Sera Sa ga a ap ae eee or —- = ER ana © 
Sy a esa asm i gee age Ss - Falta “ ng . x 7" ne PN nel OP a ae a an ee oe Pe am =e —_—~ — 
a. wees Seat Se a ee age 7 SS Sz 3 , gen ee ee Se oe era ee OS eee ete = 
ore aes np ora aie ae I a foe By Ae 2 J ae ‘ : ; os A I eg a ee gl a a ln a Oa es —— men 
ste PI wana we Pe TORS Se a nO tae Nets ee mah a cites ee of an al na ees See eo Te 1 —— eS eR ae et ee ee SS SS <i 
ee ~ being .- <— ete, Ona - aed O ~ be « ae es —_ = = PO - aoe en. = = > —_ >. eS Set seal oan me —, = Pd <_ A et ~ 
Git FS SO PO AE Ow a PPM a no eat - ——— ee an Se gat ee Pe ae SED = a Na IP “~ > 
iene wa Gt pn ag ee em SO ee era Co OOK. x . Ra ne ga eee et SE te a Se ee ae ee 
Es Re Ea a a eg mae a ann 5 ene ee a Sa ee ea ae a eae epee Sea ON eg aa agen nae pe ME ee oe tmee: 303 
rae SS Se SRE LET IO atl ah age i eee ee ean gg gg A tS eS Sc me SE S 
i ee Fa Fe es ee pe oie Oe IT So ag Se EE ge Cae Sn eI ren ae ae Sh — 
os Me, Meee tna oe -—_ Fe Pane Sse iow rea See ae as eS Any rat i cata aie Rea te teen On sie - ae = pS es 3 = 
SS ee ap ein pore ee a ee oo on i 3 Se cong nn en OE gC Ee og eS Og ee 
= RaaP aya Fe < nat in * > = ee oe a Seated ae : eweera heey in ae Ss - % - =~ > 
nos Lea a at er ga a ne ee Ee ee ke ern ee Fa an re a a I Nees a 
na | oe ee — - ae aS ame 4 — “tgp pe orig " - ee oneal Se pe ee a me, ace ee a ans “ sO genn ye 
ay Seas: FeO ae SITES yg ole eg Oe re eg at erage eet SE PR pr ee Rn at ae en i =, 
one a ol oo - sat Ce Se TF aes gt le cat a ae ON aie ye gio higcele ve ee eae ip = Se a lp ea Se ath ee aaa i re Tea 
a oe Se a ee eal i a Le perpen a eS at eee age ee a we RN a ad eS re Sparano Nae ee ee 
SA rn at Gg Sa a St eee 
or ea at al lS an Pree oe ant a a ey es th aS tte tig es ap a Ng lng a gr ae a at tt Prey, apt eh 
Sak.” dae 3 re Ae Aa bn ne ee eo 4 SS Pe FSi ee ae a eS a eer rence soe Pe en ~ 
=< ate Ten ag a a ee cB ELT mia a gO — Sars Ore I ein Pa nee ee renee toon ee ate oes cp aga —* agate, Ta 
= OT a ne Pa te nip Tg aD A cat ae Ee OR eae CFR een A gyal pgg ~o SS SNE ig ae a gee Se ee pa nn ss « 
mat TE eT er ga SF a ROO Rape ll ON ag atin. . > neo Senators Pe Se SP ee Se =e 
na 5 a nn a te ee cane Se ge nh iat need ne enh Np = ane Pps OES a" get a TE i oer gn, a a ns 
tS Se ee ee om Se a oP pee a Oe See ee 
Ee eee ig a eaeateed Pa Sp eae a I Fo a ah wenn hg” ee a cane eS eR aE ee rae St aS ee Se a a rc RS ra 
a2 3 a a ee a a np pg SI Sp ae ge ge 
SS ae a ee See eae or en a ae a eee as IE Se NOES TO ice a ete Ree ee ee 
er gS oR a Oe oat eg A cr a age ae aa ee en ar ie out . . —— a aan ad eee a ee ~ fo ag oa oe a 
ral set Na reine ste a Rhee, case ne OO wi eset NES Se an, Se oes paar ee i Oe aap eee Cage ea ee ee Se 
ow on ; nee cst ona Se ea ae aioe on Sap a ~ Sere : a 4 Se ee Teele ewe a he — 
rat nk Se PS ae i wg Sh ee Pep NE ore oe Oat Sees IN pen We te SR ae oe ee ES, ee an ae ee oe Sic a ee eS me 
aa oe * i ye ie “ ~ ~ _- “ss a = “ a ama “a wena |=. = < ee ant waste Fg eae OS ey a a = Le 
OE eS PP Dp a ag Aa Not a a8 vi . - <= Se ee Steak eee ee ae i PEO oe. a ne 
ae ate tO ae in AO, Se ag er oe ASS ae panties, Sa ee ee ee Spe ag a ne a a an 
Sy 2 ee es ee eng pe gpl er apt aN eI ng a TT Sree a le ey ee 
ro renee gr fg aes a 5 gt ale as ae iy teen A ee ee semaet SS en ie — Nee Suir An esa oe i ee a ae ee ng ete hg pene ee ee ae 
Bie, —— Fo pty 2 gt OS 5 net a, a oP runs —_ ma! - ee AS ee een el = et a ~ “~ 
wa a Ole ro aia A Se SNaR mel agetae a a le on a Ww ee a tl a <r a ese <——- 
tte oe et ‘ol Se eee ae . ee es ote oo — eee ~ sae ome = SS eS . ~~ 
A a ne ee ee a SOR RS SS 
- hes sai RE Sem ea oe ee ae al : ~ “ an a= = ae ve “. Ve - - ke es eter oe ee ee 
Taree — ee ere <i re alin pee ae ee oo a y ae; S s . “ = Se) Ae Se ee Sane varie: SKS | «a eS 
— OS Se Pt i ge aa ne “ angel m « oe Lin, Nam ed hig a ea my 
—— > Oe gt ages Ee a Pag > ie ries s ; +! 
r a i ate A Ee aS ~ eee = re Pens i & - . IT Ren 4 eee TG, Seg “OR 
so ee ee pa eae a Ne ion: 4 e ay | Poe MN a ee eee Ce YO I eRe er . 
—_ TOE LE IE PN 1 le te Ama - a Aa 8 Se PMS La > cer 
~ en a gl ON ta nay : 
“. ape nag t nell pecan tt me yg Sn ‘ a ey ah rrn a cae na af 
an ee - wite Keeedh oe ee $2 aa Cae a a: > 
ee Seen et ae yee - . ‘ 
Saee | 7 , 7 In sre pe eee ee wen , = 
v= 
























































Cia 





























THE GETTY CENTERLIBRARY 


vanes Reha! see 
i ote: e. “ i 








CELLULOSE ESTER VARNISHES 


OIL & COLOUR CHEMISTRY MONOGRAPHS 
Edited by R. S. Morrell, M.A., Ph.D., F.C. 


UNIFORM WITH THIS VOLUME 


BLACKS AND PITCHES 


By H. M. LANGTON, M.A., B.Sc., A.L.C. (Messrs. 
¥. B. Walker €3 Co., Ltd.). 


THE CHEMISTRY OF DRYING OILS 
By R. 8. MORRELL, M.A., Ph.D., F.LC. (Messrs. 
Mander Bros.), and H. R. WOOD (Messrs. Storey, 
Smithson &F Co., Lid.). 


IN PREPARATION 


THE CHEMISTRY AND MANUFACTURE OF 
PIGMENTS AND PAINTS 


2vols. By C. A. KLEIN, M.Sc. (Brimsdown Lead Com- 
pany), and W. G. ASTON (Messrs. W. Symonds & Co., 
Lid.). 


THE PROBLEMS OF PAINT AND VARNISH 
FILMS 
By H. H. MORGAN, Ph.D., B.Sc. (Messrs. Naylor Bros., 
Slough). 
VARNISH ‘THINNERS 
By NOEL HEATON, B.Sc. (Messrs. R. W. Greef &F Co.). 


THE ANALYSIS OF PIGMENTS, PAINTS 

AND VARNISHES 7 ; 
By J. J. FOX, O.B.E., F.I.C., and T. H. BOWLES, 
Fac 


RESINS: NATURAL AND SYNTHETIC 
By T. HEDLEY BARRY (Editor, Fournal of the Oil and 
Colour Chemists’ Association), and R. S$. MORRELL, 
MMA PLD, FAG 


OIL & COLOUR CHEMISTRY MONOGRAPHS 
Edited by R. S. Morrell, M.A. Ph.D., FIC. 


CELLULOSE ESTER 
VARNISHES 


BY 


F. SPROXTON, B.Sc., FLL. 
(British Xylonite Company, Ltd.) 


NEW YORK 
D. VAN NOSTRAND COMPANY 
EIGHT WARREN STREET 
1925 












Adar 


ish Siann DMS. : 


\ 


~ 




















‘ rd i 
rs ee) 
» ¥ 
‘ 
; y) 
/ > a 
- ‘ 5 
: ye 
r L 
« 
fe 4 
‘a! : . 
» > i} ’ 
ay 
Sy 
t 
pit ‘ 
1 
‘ 
é - aa 
? - a, 9 
‘ + 
~' 
r 
“ 
q ~ —_ 
3 \ 
~s 
: “2 
™ 
. 
yam hr 
: , 4 7 
\ 
:* 
hy * 
¥ ‘ *» 
, s » 
“ 
¥ . 
a ive) 
1 
j 
—_— 
wy 
Py 3 : . 
. 
Lio 
rs “ 





AUTHOR’S PREFACE 


THE primary purpose of a preface is to disarm the critic, but the 
author of a book is usually so keenly aware of its shortcomings 
that only the major criticisms can be anticipated before it passes 
into the hands of readers. Worden’s immense volumes contain 
such a complete account of the facts known about the derivatives 
of cellulose, that an author writing on the same subject, or any part 
of it, must be to some degree a plagiarist. There seems to be room, 
however, for a small book on the cellulose ester varnishes which, 
while not dealing exhaustively with any part of the subject, may 
nevertheless be individual enough in outlook to be something more 
than a guide to the larger reference books. I have thought it 
possible that such a book might be useful to the manufacturer 
of the varnishes, particularly to chemists entering the laboratories 
of such manufacturers; to the user of the varnishes, who may be 
interested to know more of the nature of, and the problems asso- 
ciated with, the materials with which he works; and, lastly, to the 
student, who may obtain a bird’s-eye view of a growing industry 
based on the principles of colloid seine and who may, perhaps, 
find some suggestions for research. 

Many references will be found to the excellent specifications 
for aircraft materials issued by the British Engineering Standards 
Association, which should be in the hands of all manufacturers of 
these varnishes. A few minor criticisms of detail will be found in 
the text of the book. The only general criticism I wish to make 
is that where the information in the specifications is derived from 
chemical literature, the principal references might be quoted. 
This practice would frequently assist the chemist who applies the 
specifications to materials in the laboratory; it would also ease the 
conscience of an author anxious not to infringe copyright. 

Patent literature has been scantily used, and then only faute 
de mieux. <A perusal of the patent literature of the cellulose esters 
generates more heat than light, and it is frequently difficult to separ- 
ate the wheat from the chaff. Wherever possible, the attention 
of the reader has been drawn to processes which have actually 
been employed on a large scale. It will be evident from the list 
of references that no single journal ministers to the entire needs 
of theindustry. The excellent abstracts of the Journal of the Society 
of Chemical Industry, Chemistry and Industry, will keep the reader in 
touch with all new developments, but the original papers must often 
be consulted. Another useful English publication is the Journal 
of the Oil and Colour Chemists’ Association. Industrial develop- 
ments can be followed in Kunststoffe, and scientific developments 

Vil 


vill Author’s Preface 


in colloid chemistry in the Kolloid Zeitschrift. Useful information 
often appears in Le Caoutchouc et La Gutta Percha, but is sometimes 
not original. 

Nomenclature is a difficult problem in a scientific book written 
on a technical subject. The word “ nitro-cellulose,’ for example, 
is scientifically incorrect ; yet it is firmly established in the industry 
and it is accepted by the learned Society which at one time used to 
alter the substantive “‘ smell ”’ to “‘ odour,” and presumably thought 
the patriarch’s description of Esau indelicate. ‘‘ Nitro-cotton ” 
is scientifically worse, but actually less objectionable (if applied 
to nitrated cotton cellulose) since it makes no pretence of scientific 
derivation. On the whole, the best plan seems to be to use the terms 
‘cellulose acetate’ and “‘ cellulose nitrate ’’ where scientific com- 
parison or contrast is drawn, and elsewhere to use the customary 
names indiscriminately. Similar considerations apply to some of 
the solvents. 

My thanks are due to the directors of the British Xylonite 
Company, Ltd., for permission to write this book; to my colleague, 
Mr. A. M. Hutchison, B.Sc., who has read the manuscript and made 
many valuable suggestions; and to the following bodies for per- 
mission to reproduce, in some instances with slight modifications, 
drawings which have appeared in their publications :— 

The Council of the Chemical Society for Figs. 3 and 4. 

The Council of the Society of Chemical Industry for Fig. 2. 

The Council of the Faraday Society for Fig. 8. 

The Aeronautical Research Committee for Figs. 5 and 6. 

The Council of the Franklin Institute for Fig. 7. 

¥.. 8, 


BRANTHAM WORKS, 
Nr. MANNINGTREE, 
Essex 


CONTENTS 


PAGE 


PREFACE : r : : : ; ; e j Vii 


CHAPTER I 


INTRODUCTION : : : : pe? 1S 


Cellulose Nitrate and Acetate—Solubility in Volatile Solvents— 
Polymerised Structure—Nomenclature of the Solutions—History of 
Cellulose Nitrate—Schonbein, Hadow, Scott Archer, Alexander Parkes 
—Stevens and Amyl Acetate—Search for Technical Solvents—Com- 
mercial Developments—Economic Considerations—History of Cellulose 
Acetate—Schiitzenberger to Cross and Bevan—Miles’s Process of 
Partial Hydration—War Expansion—Aeroplane Dopes—Shortage of 
Solvents—Post-war Developments. 


CHAPTER II 


CELLULOSE : ; ; j : : : . : ee 2 


Cellulose—Elementary Formula—Cotton Cellulose—Sources—Chemical 
Reactions—Chemical Constitution—Formula of Irvine and Hirst— 
Views of other Investigators of Cellulose—X-Ray Evidence—Viscosity of 
Cellulose and its Esters in Solution—Significance of High Viscosity. 


CHAPTER III 


NITRATION OF CELLULOSE , , .. 80 


Reaction between Cellulose and Mixed Acids—Specification of Acids— 
Acid Balance—Conditions of Nitration influencing Solubility—Ratio of 
Acid to Cellulose—Composition of Acid Bath—Time and Temperature— 
Preliminary Treatment of Cotton—Necessity for Strict Control—Varieties 
of Cotton Used—Cotton Specifications—Degree of Nitration—Nomencla- 
ture of Cellulose Nitrates—Nitration Plant—Direct Dipping—Centrifugal 
Nitration—Displacement Process—Nitration of Linters in United States 
—Removal of Acid—Boiling—Poaching—Bleaching—Drying of Nitro- 
cellulose—Dehydration of Nitrocellulose—Properties and Specifications 
of Nitrocellulose. 


CHAPTER IV 


ACETYLATION OF CELLULOSE . ; ; : eee 2: 


Reaction between Cellulose and Acetylating Bath—Differences from 
Nitration Process—Catalysts—Non-inflammable Cinema Films—Low 
Viscosity of Acetates—War Research on Dopes—Principal Manufac- 
turers and Outline of their Patented Processes—Investigations on 
Partial Hydration—Acetylation Processes in which the Acetate does 
not Dissolve—Acetylation Plant—Properties and Specification of 
Cellulose Acetate. . 


CHAPTER V 


CELLULOSE EstTER SOLUTIONS. SomE Properties. Part I. 358 


Phenomena accompanying Dissolution — Viscosity — Movement of 
Liquids in Capillary Tubes—Falling Sphere Viscometer—Dispersion 
—Solubility Limit—Solvent Power—Temperature Effect—Constitution 
of the Esters—Disintegration of Structure during Esterification— 
Fractional Precipitation of Solutions—Dimensions of Cellulose Hydro- 
lysed by Acids—Contrast with Alkaline Treatment—Chemical Con- 
stitution of Solvents of Cellulose Esters—Chiefly Carbonyl Derivatives— 
Cellulose Nitrate in Single Solvents—Baker’s Researches on Concentra- 
tion and Viscosity—Later Researches—Mixed Solvents—Acetone and 
Water—Ether and Alcohol—Cellulose Acetate in Binary Mixtures 
containing Acetone — Mardles’ Researches — Association and De- 
a of the Solvents—Plastic Flow (Bingham)—The Tyndall 
ect. J 


ix 


x Contents 


CHAPTER VI PAGE 


CELLULOSE Ester SoOLuTIONS. Some Properties. Part II. 84 


Swelling—Researches of Knoevenagel and his collaborators—Distribu- 
tion of Solvent between Swollen Ester and Liquid—Volume Change on 
Dissolution—Dielectric Capacity—Discussion of Evidence on Constitution 
of Cellulose Ester Solutions. 


CHAPTER VII 


INGREDIENTS OF CELLULOSE ESTER VARNISHES ; : ; 93 


Inflammability of Solvents—Inflammability of Coatings—Choice of 
Solvents and Diluents—Application of Laboratory Results—Variation 
in the Esters—Commercial Solvents of Esters—Approximate Classifica- 
tion—Specifications for Acetone, Methyl Ethyl Ketone, Methyl Acetone, 
Ethyl Alcohol, Methyl Alcohol and Wood Spirit, Ether, Ethyl Acetate, 
Butyl Acetate, Amyl Acetate, Ethyl Lactate, Acetone Oil, Diacetone 
Alcohol, Tetrachloroethane, Benzene, Toluene, Butyl Alcohol, Amyl 
Alcohol, Triacetin, Benzyl Alcohol, Triphenyl Phosphate, Castor Oil— 
ti nat Et a Mastic, Copal, Sandarac, Dammar—Typical Resin 
Formule. 


CHAPTER VIII 


MANUFACTURE AND APPLICATION ‘ : : / : « its 


Manufacture of the Varnishes—Mixers—Measurement of Ingredients— 
Addition of Solvents to Esters—Incorporation of Pigments—Clarification 
and Filtration—Aeroplane Dopes—Effect on Strength of Fabric—Composi- 
tion of Dopes—Doping Schemes—Method of Application—Discussion of 
Typical Dopes—Differences in Allied Practices—Protection from Sun- 
light—Metal Coatings—Application by Brushing, Dipping and Spray- 
ing — Transparent Lacquers — Enamels — Aluminium and _ Bronze 
Mediums—Motor-car Enamels—Imitation Leather—Leather Dressing— 
Patent Kid— Patent Leathers—Treatment of Motor-car Upholstery 
Leather—Coatings for Wood, Paper—Preservation of Antiques—Mis- 
cellaneous Applications—Collodion. 


CHAPTER IX 


MISCELLANEOUS E : d ; ‘ ? ‘ : . sys 


Precautions Necessary in using Cellulose Ester Varnishes—Safety Pre- 
cautions—Cause of Blooming—Humidity of Atmosphere—Measurement 
of Humidity—Effect of Raising or Lowering Temperature—Settling of 
Pigments—Cleanliness of the Work—Evaporation Losses—Analysis of 
Cellulose Nitrate Solutions; Odour, Total Solids, Nitrogen Determina- 
tions—Precipitation of Nitrocellulose with Fusel Oil (Lorenz)—Precipi- 
tation of Nitrocellulose with Chloroform (Conley)—Precipitation of Nitro- 
cellulose with Aqueous Electrolytes—Identification of Solvents— 
Viscosity—Analysis of Cellulose Acetate Varnishes—Solvent Recovery— 
Scientific Application of Cellulose Ester Solutions—Collodion Membranes 
—Interference Colours of Thin Films—Dimensions of Thinnest Obtain- 
able Films—Density of Thin Films—Chromatic Emulsions—Transport of 
Cellulose Ester Solutions on British Railways. 


INDEX . f : i : ; : : 5 ; Pa by 


LIST OF DIAGRAMS 


FIG. PAGE 
1. ILLUSTRATIONS OF SHEARING STRAIN . ; ; ‘ 59 


2. RELATIONS BETWEEN VISCOSITY, SOLVENT PowEeR NUMBER 
AND COMPOSITION, IN BINARY SOLVENT MIXTURES 


(Mardles) .. ; : een 
8. RELATION BETWEEN VISCOSITY AND CONCENTRATION OF 
SOLUTIONS OF CELLULOSE NITRATE (Baker) . , meer aS 


4. RELATION BETWEEN VISCOSITY AND WATER CONTENT IN 
ACETONE SOLUTIONS OF CELLULOSE NITRATE (Masson 
and McCall) ; ; : ; ; : ie are 


5. RELATION BETWEEN ViscosIry AND WATER CONTENT IN 
ACETONE SOLUTIONS OF CELLULOSE ACETATE (Barr and 
Bircumshaw) . : : ; ; ‘ : pga a 


6. RELATION BETWEEN VISCOSITY AND BENZENE CONTENT IN 
ACETONE SOLUTIONS OF CELLULOSE ACETATE (Barr and 
Bircumshaw) ; : : ‘ ; Peehy 


7. (a) RELATION BETWEEN PRESSURE AND OUTFLOW, AND (b) 
BETWEEN YIELD VALUE AND TEMPERATURE, OF A 
SOLUTION OF CELLULOSE NITRATE (Bingham and Hyden) 80 


8. RELATION BETWEEN TYNDALL NUMBER AND CONCENTRATION 
OF A SOLUTION OF CELLULOSE ACETATE (Mardles) eee 


xi 





CELLULOSE ESTER VARNISHES 


CHAPTER I 


INTRODUCTION 


Cellulose Nitrate and Acetate—Solubility in Volatile Solvents—Polymerised 
Structure—Nomenclature of the Solutions—History of Cellulose Nitrate 
—Schénbein, Hadow, Scott Archer, Alexander Parkes—Stevens and 
Amyl Acetate—Search for Technical Solvents—Commercial Develop- 
ments—Economic Considerations—History of Cellulose Acetate— 
Schiitzenberger to Cross and Bevan—Miles’s Process of Partial Hydra- 
tion—War Expansion—Aeroplane Dopes—Shortage of Solvents—Post- 
war Developments. 

CELLULOSE is familiar to everyone in various forms, such as cotton 
wool, cotton thread, calico, and linen. The paper on which most 
newspapers are printed contains wood pulp, an impure form of 
cellulose. Cellulose itself is not easily dissolved, and its solutions 
are not suitable for use as varnishes. Under certain conditions, 
however, it may be made to combine with acids, and the compounds 
formed are termed “ cellulose esters.”’ 

Only two esters of cellulose are of any importance in the varnish 
industry, namely, the nitrate and the acetate, formed by the com- 
bination of cellulose with nitric acid and acetic acid respectively. 
These substances are soluble in certain mixtures of organic liquids 
which have a comparatively high rate of evaporation, and when 
the solutions are spread on solid surfaces they rapidly become dry, 
leaving the cellulose ester in the form of a continuous film. By 
making certain additions to the original solutions, the hardness 
and adhesiveness of the films may be modified. Owing to the 
rapidity with which the solutions dry, they may be applied at ordinary 
temperatures, although a warm room is advantageous. As a rule 
a coating is dry in a few hours. 

All organic protective coatings consist of highly polymerised 
structures. The chief distinction between the cellulose ester var- 
nishes and the oil varnishes is that, in the former, this polymerised 
structure already exists, whereas the polymerisation of oil varnishes 
takes place to a large extent after application. The degree to which 
the original cellulose is de-polymerised or degraded in structure 
before and during the manufacture of the ester determines to a 
considerable extent the properties of the resulting varnish. 


Properties of Cellulose Ester Varnishes. 


The cellulose ester varnishes occur in commerce as more or less 
viscous solutions, which may be either transparent or opaque. 


The transparent varnishes may be coloured in practically any tint, 
13 


14 Cellulose Ester Varnishes 


so as to give a coloured transparent coating.» These are usually 
termed lacquers. The opaque varnishes may also be obtained 
in any colour and are usually termed enamels. As a rule they are 
made so as to dry with a bright surface, but the enamels are some- 
times required to dry with a dull or matt surface. It is becoming 
customary to include all solutions of cellulose esters used for pro- 
tective or decorative coatings under the general term of “ cellulose 
varnishes.””’ The name is not a happy one, as the materials do 
not resemble the oil varnishes very closely, and they are not solutions 
of cellulose. To those accustomed to scientific terminology, “‘ cellu- 
lose ester varnish”’ and “cellulose ester solution ’’ would bear 
somewhat similar meanings, but “ cellulose solution ’’ would denote 
something very different. However, it is difficult to dislodge a 
trade name, which in this instance probably dates back to the 
discovery of “‘ cellulose ”’ by the public in 1916. 

As all these varnishes are made with volatile organic solvents, 
they have characteristic smells which disappear when the coating 
dries. They may be applied by brush, but better results are obtained 
by dipping or by use of an air-spray. Under proper conditions, 
the solutions dry with a high polish, and the films resist water, 
petrol, soap, turpentine, weak acids and weak alkalies. 


Cellulose Nitrate Solutions—Historical. 


Although. cellulose has been known from time immemorial, 
the history of cellulose nitrate solutions scarcely begins until the 
latter half of the nineteenth century. Pelouze,! in 1838, investi- 
gated the action of nitric acid on paper, linen, and cotton wool, but 
the importance of the products was first recognised by Schénbein,? 
who in 1845—6 prepared nitrates of cotton cellulose by a process 
which he kept secret. Otto? discovered and published the method 
of production a little later, and Knop 4 is said to have been the first 
to make use of a mixture of sulphuric and nitric acids, thereby 
laying the foundation of the modern cellulose nitrate industry. 
The papers in which these early discoveries are described are avail- 
able in an English translation by MacDonald.’ Schénbein and his 
collaborator Béttger, although they observed the solubility of their 
product in various organic liquids such as ethyl acetate, were chiefly 
interested in it as a substitute for gunpowder. An interesting 
review of the current state of knowledge of cellulose nitrate is 
given in Gmelin’s “ Handbook of Chemistry ” (1872 edition),® 
although no notice is taken of such technical developments as those 


Introduction 15 


of Parkes (v. infra). It was known by then that the properties 
of the product varied largely according to the composition of the 
acid mixture, the time and the temperature of nitration. Directions 
are given for the preparation of (a) an explosive pyroxylin, (b) a 
collodion-wool soluble in ether-alcohol, and for the preliminary 
treatment of the cotton with boiling soda solution and bleach. 
Various analyses of the product, by Schénbein, Crum, Pelouze, 
Gladstone and others, are given, showing percentages of nitrogen 
ranging from 9-3 to 14:26, while among the solvents quoted are 
ether—alcohol, ether alone, alcohol, ethyl acetate, glacial acetic acid, 
acetone, and wood spirit. The first important researches on the 
solubility of cellulose nitrate were carried out by Domonte and 
Ménard ? (1846), who investigated the difference between ether- 
soluble and ether-insoluble products. Hartig § in 1847 stated that 
cellulose nitrate formed dark brown solutions with common camphor, 
oils, waxes and resins. This appears to be the first occasion on 
which cellulose nitrate and camphor were brought together, but his 
observations apply to heated mixtures and not to any product 
resembling either a modern celluloid varnish or solid celluloid. 

The researches of Hadow ® in 1855 on the relation between the 
composition of the acid mixture and the properties of the cellulose 
nitrate produced are of interest as the first of a long series of similar 
experiments by various chemists. The value of the observations 
recorded is in many cases discounted by the fact that the cellulose 
which formed the starting point is not rigidly defined. This aspect 
of the nitration of cellulose will be briefly developed later in the book. 

The first attempts to find practical applications for solutions 
of cellulose nitrate were made in 1848 by Maynard ?° and Bigelow,}! 
who suggested independently the use of a solution of cellulose nitrate 
in ether—alcohol (collodion) as a protective dressing for wounds. 
F. Scott Archer 12 in 1851 was the first successfully to use collodion 
in photography, and there is an interesting description of the pre- 
paration and use of collodion for this purpose in Robert Hunt’s 
“Manual of Photography’ (Griffin, 1853).* All of the early 
workers were confronted with the difficulty that these solutions 
contracted considerably on drying, causing puckering and uneven- 
ness, while at times the films would be more or less white and opaque 
instead of transparent. 

Alexander Parkes, the greatest of the pioneers in the develop- 
ment of the industrial uses of cellulose nitrate, began work on the 


; * The writer is indebted to Mr. A. M. Hutchison for the loan of this 
ook | 


16 Cellulose Ester Varnishes 


subject in the early ‘fifties. Although concerning himself chiefly 
with the manufacture of plastic solids, he investigated many liquid 
solvents and foresaw the value of the material as a base for varnishes. 
He recognised that the opacity frequently manifested by films from 
ether-alcohol solutions was due to precipitation of amorphous 
cellulose nitrate by absorption of atmospheric moisture, and was 
the first to employ anhydrous solvents. An early patent of his 
describes the dehydration of wood spirit (crude methyl alcohol) 
by distillation with calcium chloride.* This patent makes special 
reference to the use of solutions as protective coverings, and fore- 
shadows many later developments, thus: “ For a transparent 
and colourless substance I use a solution of guncotton alone or with 
gums or resins that will set transparent with it and this is applicable 
as a coating to silk tinsel articles or other textile fabrics, sewing 
cotton thread, worsted, string, felted goods, leather, plaster or wood.”’ 
In the same specification Parkes also suggests the use of metallic 
bronzes with cellulose nitrate solutions. He recommends using 
thick solutions and spreading machinery, thereby producing films 
which can be used for bookbinding or button manufacture. Lastly, 
in order to render the film non-inflammable he recommends the 
incorporation of ammonium phosphate, magnesium phosphate, 
cadmium iodide, mercuric iodide, calcium oxalate, talc or alum. 

Parkes realised that satisfactory films would not be obtained 
under ordinary atmospheric conditions from solutions of cellulose 
nitrate in solvents of low boiling point, and he investigated aniline 
and nitrobenzene. These, however, failed for the opposite reason, 
their vapour pressure being so low at ordinary temperatures that 
solutions would not dry in a reasonable time. The poisonous 
nature of the vapours was also a drawback. Acetic acid was 
obviously unsuitable on account of its unpleasant and corrosive 
vapour. 

In 1864 Parkes 1 disclosed the strong solvent power possessed 
by solutions of camphor in oil of turpentine or in wood spirit. He 
was therefore the first to use camphor in useful conjunction with 
cellulose nitrate, and in a lecture given before the Royal Society 
of Arts in 1865 he describes the use of camphor as an important 
improvement in the manufacture of solid masses from pyroxylin, 
since it renders them more uniform and less contractile. 

Pellen’s patent of 185615 for the use of collodion for covering 
toys and balloons is of interest in view of the immense Maino 6i3 
of aircraft dopes sixty years later. 

The most interesting of Spill’s 1° patents in reference to besa 


Introduction 17 


of cellulose nitrate is that in which he makes known the solvent 
power of a mixture of ethyl alcohol and benzene, each constituent 
being a non-solvent when employed alone. The study of these 
mixed solvents is one of the most interesting problems in the chemistry 
of cellulose nitrate. A mixture of ether and alcohol is one of the 
commonest solvents of cellulose nitrate, containing from 11:0 to 
12-4% nitrogen, although neither constituent alone is a solvent at 
ordinary temperatures. It was employed by nearly all the early 
experimenters and was investigated by Saillard in 1859.17 A care- 
ful but inconclusive research was carried out by Baker 18 in 1912, 
while as recently as 1921 Bancroft ! has described it as one of the 
present problems of colloid chemistry. 


Amyl Acetate. 


Until the year 1882, the development of a large industry in 
solutions of cellulose nitrate was prevented by the fact that there 
were no cheap and otherwise desirable solvents and solvent mixtures 
known intermediate in boiling point between the group of low- 
boiling liquids such as methyl alcohol, acetone, ethyl acetate, ethyl 
alcohol, ethyl ether and benzene, and the high-boiling slow-drying 
solvents such as aniline and nitrobenzene. The gap was filled in 
1882 by the discovery of the solvent power of amyl acetate 
by Stevens.” Crude amyl alcohol, the so-called fusel oil, is a by- 
product in the production of ethyl alcohol by fermentation, and for 
many years was of so little value that it was burned in lamps in 
the distilleries in place of paraffin oil. Stevens found that the ace- 
tate of amyl alcohol (strictly speaking, of the mixture of various 
amyl and other alcohols which fusel oil contains) possessed the blend 
of properties required for his purpose, namely, high solvent power 
for a large range of cellulose nitrates, a vapour pressure at ordinary 
temperatures low enough to prevent unduly rapid evaporation 
but high enough not to prolong the drying of the film, and very small 
affinity for water. 


Search for Technical Solvents. 


The search for solvents of cellulose nitrate has continued right 
down to the present day, but since the discovery of the solvent 
power of amyl acetate there have not been very many additions to the 
list which have had much influence on the varnish industry. The 
acetins (acetates of glycerol) were patented by Schiipphaus in 1889.7} 
F, oe 22 patented the use of acetone oil (a mixture of indefinite 


18 Cellulose Ester Varnishes 


composition remaining from the fractionation of acetone) in 1892, 
and a large number of compounds, including some esters which have 
had a limited application in the varnish industry, were patented 
by A. Nobel, from 1889 onwards. 

The earliest commercial development of cellulose nitrate var- 
nishes took place in the United States. The Stevens patents 
belonged to the Celluloid Company of New Jersey, while Hale and 
Crane formed the Crane Chemical Company and began the manu- 
facture of varnishes and lacquers in 1886. The usefulness of the 
lacquers is greatly increased by the incorporation of resins, such as 
shellac, mastic and copal. Experiments in this direction date back 
at least to the 1855 patent of Parkes already quoted, but the first 
products of the type of modern lacquers, in which cellulose nitrate 
is blended with a resin in a mixture of solvents of graded volatility, 
were due to Field.?8 Such varnishes are more adhesive than those 
prepared with cellulose nitrate alone, and can be made with a 
higher solid content for the same degree of fluidity. At the present 
time they form a large proportion of the total output. 


Economic Considerations. 


In deciding the suitability of any substance as an industrial 
solvent for cellulose nitrate, economic considerations must come 
first. So much is known about the chemical groupings which impart 
solvent power to an organic molecule that it is safe to say an expert 
organic chemist could, if it were worth while, make extensive 
additions to the list of cellulose nitrate solvents; but only those 
substances which can compete in price with the known solvents 
have any chance of industrial application. Occasionally the develop- 
ment of chemical industry may result in a group of solvents, hitherto 
of academic interest only, becoming industrially available. For 
example, the fermentation of starch to butyl alcohol and acetone 
was originally developed to yield butyl alcohol for the manufacture 
of artificial rubber.24 The acetone was absorbed in the explosives 
and varnish industries. During the Great War, acetone became the 
more important product, and as artificial rubber became of negligible 
account in England, butyl alcohol and butyl acetate became available 
as substitutes for amyl alcohol and amyl acetate. Since amyl 
acetate tends to become increasingly scarce, owing to the demands 
of the film and varnish industry and the diminution of spirit manu- 
facture in the United States, butyl acetate has become a normal 
solvent in the cellulose ester industry. 





f 
J 


Introduction 19 


Unless, however, the discoverer of a new solvent for celluiose 
esters sees some probability of the material being available at a price 
not greater than that of the usual solvents, it is not worth while 
for him to seek patent protection. — 


Cellulose Acetate—Historical. 


Shortly after the time when Parkes began working on the indus- 
trial possibilities of cellulose nitrate, another industrial ester of 
cellulose, the acetate, was first prepared by Schiitzenberger.?5 
The principal difference between the nitration and the acetylation 
of cellulose is that whereas cellulose nitrate is insoluble in the 
“spent acid,” 7.e., the weakened mixture of sulphuric and nitric 
acid remaining after nitration of the cellulose, cellulose acetate is 
soluble in acetic acid. Hence, unless special indifferent diluents 
are added to the acetylation mixture, the cellulose passes into solu- 
tion as acetylation proceeds, yielding a viscous mixture from which 
cellulose acetate is precipitated by the addition of water. In 
Schiitzenberger’s first paper, he described the preparation by heating 
cellulose with acetic anhydride in sealed tubes to 130—140°C. In 
a later publication he recommended a temperature of 180°C., 
obtaining a product insoluble in alcohol or ether, and easily saponified 
by caustic alkalies. 

The use of acetic anhydride under pressure was avoided by 
Franchimont’s discovery of the so-called catalytic action of sul- 
_ phuric acid or zinc chloride.26 This step really marks the foundation 
of the modern cellulose acetate industry, Cross and Bevan used 
the zinc chloride method in their investigations of the constitution 
of the ligno-celluloses.2” These investigators took out a patent in 
1894 2° for a method of acetylation involving the use of zinc chloride. 
The product was soluble in chloroform, yielding films which were 
suggested as substitutes for collodion in surgery. It is with this 
patent that the industrial history of cellulose acetate may be said 
to begin. 

The technical development of cellulose acetate was much hindered 
by the slowness of the reaction between cellulose and the acetylating 
agents, and the limited range of solubility of the acetates produced. 
Investigators were therefore led to experiment on the acetylation 
of modified forms of cellulose, 7.e., cellulose which has been rendered 
more open to attack by partial degradation of its structure. Girard 2° 
had applied the name of hydrocellulose to the derivatives obtained 
by the mild acid hydrolysis of cellulose, and Lederer *° found that 


20 Cellulose Ester Varnishes 


hydrocellulose could be acetylated more smoothly and at a lower 
temperature than unmodified cellulose. F. Bayer & Co. #} 
patented a similar process almost at the same time. 

So far the cellulose acetates produced had not been soluble in 
any of the useful organic solvents unless hydrolysis had been carried 
so far that the films left on evaporation were so brittle as to be 
technically useless. A great advance was made by Miles,*? who 
found that by a partial hydrolysis of the primary product of acetyla- 
tion, an acetate soluble in acetone was produced. In 1906 the 
Bayer Co.** patented an adaptation of Miles’s process, and in 1907 
they placed on the market varieties of cellulose acetate described as 
Cellite L and Sericose. These produced moderately good films when 
evaporated on glass, and were intended as textile finishes. The 
solvents employed for Cellite L were acetone or a mixture of ethyl 
acetate and alcohol. Sericose was insoluble in anhydrous acetone, 
but soluble in aqueous acetone or in acetic acid; either concentrated 
or somewhat dilute. It is evident that these substances were acetyl 
derivatives of a highly depolymerised cellulose. The acetic acid 
was not firmly held, as both the solid materials and their solutions 
rapidly developed a smell of free acetic acid. Owing to their high 
price (about seven shillings per pound) they never came into common 
use. 

Kichengriin discovered the solubility of certain varieties of cellu- 
lose acetate in a hot mixture of benzene and alcohol and did much to 
develop the acetate varnishes. The later history of the acetate and 
its manufacture on a commercial scale by the Usines du Rhone, 
Dreyfus, and F. Bayer & Co., will be briefly sketched in a later 
chapter. 


War Expansion. 


An enormous development in the use of cellulose ester varnishes 
was brought about by the Great War of 1914—18, particularly in 
regard to cellulose acetate solutions. It had been found previous 
to the war *4 that these solutions, when applied to textile fabrics 
stretched on a frame, possessed the valuable property of shrinking 
and drawing the fabric taut. This property was utilised for making 
taut the wings of aeroplanes and protecting the fabric from the 
weather. It was subsequently found that by adding appropriate 
ingredients to the solutions the destructive effect of bright sunlight 
on the fabric could be considerably diminished. Similar effects can 
be produced with solutions of cellulose nitrate, but since the total 
thickness of the coatings applied to the fabric is quite considerable, 


Introduction 21 


and the fabric itself is combustible, the use of cellulose nitrate 
alone adds appreciably to the fire danger, especially when the machines 
are used on active service. 

Cellulose acetate was not manufactured in this country before 
the war, and the needs of the Government after the outbreak of war 
were met for a time by the importation of the material, chiefly from 
Switzerland and France. This arrangement, however, had finally 
to come to an end, owing to the great expansion in the demand, 
and a factory for the manufacture of cellulose acetate base and 
varnishes was established with Government assistance near Derby. 
Since the conclusion of the war, the demand for cellulose acetate 
dopes has naturally diminished, and the manufacturers have given 
their chief attention in this country to the production of artificial 
silk (rayon) from cellulose acetate. 

This, however, was by no means the end of the difficulties. 
The principal solvent for cellulose acetate was acetone, a product 
of the distillation of wood, and during the early part of the war 
acetone was needed, not only for cellulose acetate varnishes, but 
also for the still more important material cordite. Certain wood 
distillation factories were established in this country, and the 
fermentation process by which acetone (and butyl alcohol) are pro- 
duced from starch was developed. Nevertheless, the bulk of the 
country’s requirements in acetone was imported, and since the 
demand exceeded the supply, and there were many other claims 
on the available shipping capacity, it became imperative to restrict 
the use of acetone as much as possible. Hence a great deal of research 
was carried out in two or three years which would probably have 
been spread over many years in times of peace. These researches 
had for their object to find the most economical way of using 
imported solvents, and involved numerous investigations into the 
changes of solvent power caused by mixing different liquids together, 
and into the resulting variations in viscosity of solutions of cellulose 
acetate made up from the mixtures. 

In the explosives industry, the shortage of acetone required 
for cordite was still more severely felt, and led in time to the radical 
alteration of service cordite.*> Instead of guncotton, which is a 
nitrate of cellulose containing about 13-0°% of nitrogen, soluble in 
acetone but insoluble in any other available low-boiling solvent, 
the new cordite (R.D.B.) was made from a collodion cotton, con- 
taining about 12-4% of nitrogen, soluble in a mixture of ether and 
alcohol, and resembling a varnish nitro-cotton. Since ether is 
made from alcohol, and alcohol is cheaply and easily made from 


22 Cellulose Ester Varnishes 


molasses or any material containing starch, the use of acetone 
or any other product of the distillation of wood in the manufacture of 
cordite was thus entirely avoided. This change of solvent, however, 
involved, not only investigations into the best proportions of ether 
and alcohol to be used as the solvent, but also fundamental work 
on cotton cellulose itself, since it was found that cotton from different 
sources might give nitrates so different in solubility that not only 
the consumption of solvent but also the output of the new cordite 
were gravely affected. 

The war also caused a considerable expansion in the use of 
solutions of cellulose nitrate for cementing the joints of celluloid 
accumulator cases, and for fixing glass to celluloid in the eyepieces 
of gas masks. 

Since the war, some of the information gained in the Government 
factories has been published, and is influencing the development 
of industry. The most interesting advance is in the use of cellulose 
nitrate solutions of much greater fluidity than formerly. It must 
be understood that the high viscosity of these solutions imposes 
a practical limit on the amount of cellulose nitrate that can be 
dissolved in the solvents, since it is obvious that they must be fluid 
enough to flow somewhat like a paint. By preparing cellulose 
nitrate which yields solutions of greatly increased fluidity, it is 
possible to dissolve more of the ester than before without exceeding 
this practical limit, and a varnish is thus obtained which yields a 
much thicker coating for the same expenditure of labour and solvent. 
These very fluid cellulose nitrate varnishes have been developed 
chiefly in the United States, but the process of manufacture has 
been kept secret.?® 


REFERENCES AND BIBLIOGRAPHY. 


1 Pelouze, Comptes rend., 1838, 7, 713. 2 Schdénbein, Pogg. Ann., 1847, 
70, 320. * F. Otto, Augsburger Allgem. Zeitung, 1846. * Knop, Comptes 
rend., 1846, 23, 808. 5 MacDonald, ‘‘ Historical Papers on Modern Ex- 
plosives,” 1912. ® Gmelin, ‘‘ Handbook of Chemistry,’ 1872, Vol. XV., 
168-181. 7 Domonte and Ménard, Comptes rend., 1846, 28, 1087 and 1847, 
24, 87, 390. &® Hartig, ‘ ‘ Untersuchungen tiber ‘Schiessbaumwolle, ”* 1847. 
9 Hadow, Chem. Soc. Quarterly Journal, 1855, 7, 201. 1° Maynard, Boston 
Med. and Sci. J., 1848, 38, 266. 11 Bigelow, ibid., 1848, 38, 178. 12 F. 
Scott Archer, The Chemist, New Series, 1851, 2, 257. 13 A. Parkes, E.P. 
2359/1855. 14 A. Parkes, E.P. 2675/1864. 15 Pellen, E.P. 2256/1856. 
16D), Spill, E.P. 1739/1875. 27 Saillard, E.P. 444/1859. 18 F. Baker, 
Chem. Soc. Trans., 1912, 102, 1409. 1% W. D. Bancroft, J. Ind. Eng. Chem., 
1921, 13, 260. %° J. H. Stevens, U.S.P. 269,340. #1 R. Schiipphaus, U.S.P. 
410,208/1889. 2? F. Crane, E.P. 6543/1892. °° W. D. Field, U.S.P. 
422,195/1890. #4 W. H. Perkin, jun., J. Soc. Chem. Ind., 1912, 31, 616-624. 
25 Schiitzenberger, Comptes rend., 1865, 60, 485-486. 26 A. Franchimont, 
Comptes rend., 1879, 89, 711; ibid., 1881, 92, 1053. 27 Cross and Bevan, 





Introduction 28 


Chem. News, 1889, 60, 163, 254. 8 Cross and Bevan, E.P. 9676/1894. 
29 Girard, “‘ Mémoire sur |’Hydrocellulose et ses dérivées,’? Paris, 1881. 
80 Lederer, G.P.’s 118,538/1899 and 120,713/1900. °1 F. Bayer & Co., 
F.P. 317,007/1901. %* G. W. Miles, U.S.P.’s 733,729/1903 and 838,350/1905. 
33 FY. Bayer & Co., E.P. 24067/06. 34 J. E. Ramsbottom, ‘‘ Tech. Report 
of Advisory Comm. for Aeronautics,’”’ 1913-14, p. 426. %5 R. Robertson, 
Trans. Faraday Soc., 1921, 16, 66-71. °° G. E. Condé, Canad. Chem. and 
Metall., 1924, 8, 219-221. | 

See also E. Fischer, Kunststoffe, 1912, 2, 21, 48, 64; 1914, 4, 102, 123 
(chronological list of patents and brief abstracts). Clément et Riviére, <bid., 
1914, 4, 148-152, 166-168 (translated from Comptes rend. with annota- 
tions by W. Vieweg). C. P. Schwalbe, J. Soc. Dyers and Colorists, 1914, 
30, 13-15. E. C. Worden, J. Soc. Chem. Ind., 1919, 38, 370-374r. P. 
Drinker, J. Ind. Eng. Chem., 1921, 18, 831-835; ‘‘ European Practice in 
Cellulose Acetate and Dopes during the War.”’ Abel Caille, Chim. et Ind., 
1924, 12, 441-448 (an historical review of the manufacture of cellulose 
acetate). E.C. Worden, “‘ Technology of Cellulose Esters,’’ Vol. I., Pt. IV. 
E. C. Worden, ‘“ Nitrocellulose Industry,” Vol. I., chap. x. F. Ullmann, 
**Enzyklopadie der technischen Chemie,”’ Vol. III., 309-316. Clément et 
Riviére, “‘ Matiéres Plastiques et Soies Artificielles ’’ (Bailliére et Fils), 1924, 
Pt. I., chap. ii.; Pt. I1., chaps. i. and ii. 


CHAPTER II 


CELLULOSE 


Cellulose—Elementary Formula—Cotton Cellulose—Sources—Chemical Re- 
actions—Chemical Constitution—Formula of Irvine and Hirst—Views 
of other Investigators of Cellulose—X-Ray Evidence—Viscosity of 
Cellulose and its Esters in Solution—Significance of High Viscosity. 


In this volume it is only necessary to discuss cellulose in relation 
to its esters and the solutions made from them. Cellulose may be 
described as the chief constituent of the skeleton of all vegetable 
bodies. Elementary analysis shows it to belong to the group of 
organic bodies known as the carbohydrates, and to have the com- 
position C,H,,0;. Its physical and colloidal characteristics indicate 
that we must regard it as a polymer of the simple formula, and it 
is usually written as (C,H, )0;)n. The latest researches on its 
constitution show that there may be two varieties of polymerisation 
in the cellulose structure, so that we might write it as [(C,H,)0;)n|m, 
n being probably a small integer and m a large and probably variable 
number. 

The kind of cellulose used commercially for the manufacture 
of esters is almost entirely cotton cellulose, although wood cellulose 
has been used, particularly for the manufacture of cellulose nitrate, 
when supplies of cotton have been short } 2%. Cotton is the purest 
form of cellulose produced by nature in large quantities, and con- 
tains about 90% of cellulose. The coniferous woods which form 
the source of wood cellulose contain about 60°%,4 and therefore 
require a greater degree of purification to isolate the cellulose. 

Cotton is cultivated industrially chiefly in the United States 
and in India. The cotton fibres or hairs are attached to seeds 
contained in the “‘ boll”’ or seed-pod of the cotton plant. When 
the bolls ripen they burst open, and the white mass of cotton fibre, 
after being freed from the seeds, forms the raw material of the 
spinning industry. 

The chemical reactions which the constitutional formula of 
cellulose must explain are conveniently summarised by Green 5 
and are quoted by Hall.* It is not necessary to give the complete 
list here, but the following properties have a bearing on the specifica- 
tion of cotton and its use in the cellulose ester industry :— 


(a) The highest stage of nitration or acetylation is a tri- 
ester calculated on a C, formula. 

(b) Cellulose does not contain free carbonyl (aldehydic 
or ketonic) groups, but such groups are formed by simple 
hydrolysis. 

~ 24 


~ 


Cellulose 25 


(c) The ultimate product of hydrolysis is dextrose. (The 
rigid proof of this is much more recent.) 

(d) The oxidation of cellulose yields oxycellulose, which 
is markedly acidic. 


The proof of the chemical constitution of cotton cellulose has 
been given by Irvine and Hirst,’ based principally on their own work, 
that of Denham and Woodhouse,’ and of Haworth and Leitch.® 
The original papers must be read for a detailed account. The 
methods used were chiefly— 


(1) hydrolysis and graded acetolysis (i.e. simultaneous 
hydrolysis and acetylation) of cellulose, and 
(2) alternate methylation and hydrolysis. 


The researches involved identification of the products of these 
reactions, and careful determinations of the yields in which they 
were obtained. 

It was shown that cellulose contains the dextrose grouping 


and next that the residues B and C were also themselves entirely 
composed of groups having the dextrose configuration. There are 
several ways in which such a structure might be built up, but Irvine 
prefers the symmetrical formula :— 


CH,OH 


| ee Oe 
-CH—0—CH—CH—CHOH—CHOH—CH 


j (HOH 


eyes O 


Sa ae a 
HH '————-o————_~ 


It may be noted that one step in the proof depends on the isolation 
from cellulose by Haworth and Hirst of a crystalline derivative 
of a sugar containing two C, groups in the molecule (cello-biose). 
It will be seen that Irvine’s formula could only yield one molecule 
of a cello-biose (C,,) from one unit (C,,) of cellulose, and calculation 
shows that the maximum yield of cello-biose from cellulose would 


26 Cellulose Ester Varnishes 


be 70%. The highest recorded yields are 50—60%, and a yield 
of more than 70% would disprove Irvine and Hirst’s symmetrical 
C,, formula. Irvine and Hirst exclude the possibility of cello- 
biose being formed in a secondary reaction by the condensation 
of two molecules of dextrose. 

The formula represents cellulose as possessing three hydroxyl 
groups for every six carbon atoms. Hence unless cellulose is hydro- 
lysed (with formation of new hydroxyl groups) only three ester 
radicals can be introduced into each C, unit. It may be remarked 
in passing that two of the hydroxyl groups are present in the form 
of secondary alcoholic groups, and the other as a primary alcoholic 
eroup.!° The formula does little, however, to explain those pro- 
perties of cellulose and its derivatives which are of chief interest 
to the varnish manufacturer. Why, for example, is cellulose, with 
its high percentage of hydroxyl groups, insoluble in water? Why 
have cellulose and its esters such a high viscosity in solution ? 
Irvine and Hirst realised these difficulties, and their own words 
may be quoted. The formula is “the simplest expression of a 
molecule which, polymerised in unknown numbers, would represent 
cellulose as a chemical entity, but we recognise that any odd number 
of anhydroglucose residues from three upwards can be arranged so 
as to fulfil the conditions. Our work gives no indication of the degree 
of polymerisation undergone by the molecular unit, but the extreme 
insolubility of trimethyl! cellulose compared with the ready solubility 
of methylated starch, inulin and glycogen points to the idea that 
cellulose is the most highly polymerised of the polysaccharides.” 
It has been remarked by Dorée that the purely chemical and the 


colloidal investigations of cellulose have proceeded almost indepen- — 


dently, and it is of interest to place next to the foregoing quotation 
from Irvine the views of some of those who have been chiefly 
interested in cellulose as a colloid. 

W. Harrison 1! suggests that the cellulose fibre itself may be 
regarded as the molecule. Schwalbe and Becker 1* found that when 
cellulose is mechanically converted into a mucilage by minute 
subdivision in water, its power of reducing copper is materially 
increased, presumably because the mechanical rupture of linkages 
exposes fresh oxidisable atomic groupings. W. L. Balls ® by the 
microscopic examination of sections of swollen cotton fibres was 
able to show well-defined growth rings, and in a later paper 
concludes that the wall of the hair is probably a sponge-like structure 
with free air-spaces. Pierce has measured both the rigidity 15 and 
the plasticity 1° of the cotton fibre. On the other hand, C. F. Cross 1 


Cellulose 27 


regards the cellulose fibre as resembling a liquid system : “‘ Cellulose 
(reacts) as a labile complex of groups of varying dimensions repre- 
senting a state of matter somewhat analogous to that of a solution 
of a saline electrolyte—that is, it reacts rather as a solution—aggre- 
gate than by a succession of molecular combinations; the masses 
actually reacting following the stoichiometrical ratios proper to 
the dimensions of these ultimate groups, and retaining their relation- 
ships in the aggregate, which is thus progressively modified by the 
entrance of the new groups.” 

S.S. Napper 1° regards Cross’s suggestion as tending in the same 
direction as Harrison’s, and as being in harmony with the view that 
cellulose is built up by the condensation of soluble constituents of 
plant juices. J. Boeseken, J.C. van den Berg and A. H. Kerstjens,?9 
from a study of the graded acetylation of cellulose, concluded that 
if the formula is written as (C,H,,)0,;),, ~ must be at least 45. 
Duclaux and Wollmann *° fractionally precipitated an acetone 
solution of cellulose nitrate and obtained fractions of identical 
elementary composition but of such widely different viscosity that 
they probably represented large variations in molecular size. 

Any theory of cellulose structure built up from Irvine and Hirst’s 
formula must take account of these views. The polymerisation 
of the units during growth must take place in three dimensions, and 
it is not difficult to imagine that the method of condensation may not 
be uniform. There may be denser packing of C, units in some 
parts of the same fibre than in others, the density depending on 
atmospheric conditions during growth, such as temperature and 
humidity, which would probably influence the degree of internal 
anhydride formation. On the other hand, we must assume that 
the structure is sufficiently open to allow penetration of cupram- 
monium solvent, for example, which under controlled conditions 
will dissolve cellulose without degradation of the structure, or of 
nitrating acid which will convert cellulose into its nitrate with com- 
paratively little degradation. It may be that the somewhat con- 
flicting evidence on the yield of cello-biose obtainable from cellulose 
is due to the fact that the condensation of C, groups in the structure 
proceeds irregularly. 


Rontgen Ray Evidence. 


For work on the application of von Laue’s X-ray method to the 
elucidation of the structure of cellulose, reference must be made to 
the papers of Herzog and Jancke.?!_ These investigators state that 
cellulose from cotton, ramie and wood all showed evidence of con- 


28 Cellulose Ester Varnishes 


taining a crystalline substance with rhombic symmetry, with ratios 
a:b:c = 0-6935:1:0-4467. There is also evidence that the 
particles are regularly orientated or arranged in the direction of the 
length of the fibre. The dimensions of the elementary cellulose 
particle are found to be 7:9: 8-45: 10:2 x 10°§ cm., and Herzog 
and Jancke deduce that it contains four dextrose residues, or two 
cello-biose residues. Irvine and Hirst, from chemical evidence, 
prefer to assume three dextrose residues, or one cello-biose and one 
dextrose residue, in the unit. 

It should be explained that the terms crystal and crystalline 
refer here solely to the existence of a repeated regular structure in 
cellulose, causing it to produce a regular pattern by the diffraction 
of X-rays. The popular criteria of the crystalline state, 7.e., visible 
geometrical uniformity, some degree of hardness, and unscattered 
reflection of light, are evidently absent. 


Significance of Viscosity of Cellulose Ester Solutions. 


The most important property of cellulose and its esters from the 
point of view of the varnish manufacturer is the viscosity of their 
solutions. This property, which is the inverse of fluidity, is a 
measure of their resistance to flow. It will be treated more fully 
in a later chapter, but it is necessary to clear up at this point a 
misapprehension that may arise. It is often stated that the vis- 
cosity of cellulose ester solutions is an indication of a highly polymer- 
-ised molecular structure, and that degradation of the structure 
reduces viscosity and vice versa. On the other hand, it will be shown 
later that the same sample of cellulose ester will yield solutions of 
very different viscosity in different solvents and combinations of 
solvents. It is natural to ask whether the degree of polymerisation 
of the structure differs in different solvent media? A full answer 
to this question cannot be given until the constitution of these 
colloidal solutions has been cleared up, but it may be taken as cer- 
tain that the difference in viscosity between solutions of the same 
sample of ester in different solvents does not indicate a difference 
in the molecular structure of the ester particles, but in the degree to 
which they are surrounded and swollen by solvent molecules. It 
follows that it is not safe to draw conclusions as to structural degra- 
dation of cellulose esters from viscosity determinations in any 
single solvent. To put this perhaps more correctly, it is not safe 
to deduce a high degree of polymerisation from the fact that a 
sample of ester has a high viscosity in one particular solvent or mixture 


Cellulose 29 


of solvents. From the fact that acetone dissolves a very wide range 
of the nitrates and acetates of cellulose, it has become almost 
customary to draw conclusions as to the magnitude of the molecular 
structure from viscosity determinations in acetone solutions. It 
would be safer to employ a range of solvents, including aqueous 
acetone and other binary mixtures, and to base deductions as to 
the structure on the lowest viscosity measured. 


REFERENCES AND BIBLIOGRAPHY. 


1 C. G. Schwalbe and A. Schrimpf, Z. angew. Chem., 1914, 27, 662. 
2 B. Rassow, zbid., 1924, 37, 792. *% G. Leysieffer, Koll. Chem. Beth., 1918, 
10, 145. 4 A. J. Hall, ‘‘ Cotton Cellulose,’ 1924, 25. 5 A. G. Green, Zeit. 
Farben u. Textil.-Chem., 1904, 3, 97. & A. J. Hall, loc. cit., pp. 182-183 (see 
also H. Hibbert, J. Ind. Hing. Chem., 1921, 18, 256-260, 334-342). 7 J.C. 
Irvine and E. L. Hirst, Chem. Soc. Trans., 1923, 123, 518. 8 W.S. Denham 
and H. Woodhouse, ibid., 1913, 108, 1735. ® W. N. Haworth and G. C. 
Leitch, 7b7d., 1918, 113, 188. 1° H. Hibbert, loc. cit.; also J. Amer. Chem. 
Soc., 1923, 45, 734. 11 W. Harrison, 2nd Report on Colloid Chemistry, 
1919, 55. 12 C. G. Schwalbe and E. Becker, Z. angew. Chem., 1919. 32, 
265. 4 W. L. Balls, Proc. Roy. Soc., 1919, B90, 542. 14 Idem, ibid., 1923, 
B95, 72. 15 F. T. Pierce, J. Text. Inst., 1923, 14, lr. 1° Idem, ibid., 390r. 
17 C. F. Cross and E. J. Bevan, ‘‘ Researches on Cellulose,’’ 1900-1905, 7. 
18 §. S. Napper, ‘“‘ Applied Chemistry Reports,’ 1919, 121. 1% J. Boeseken, 
J. C. van den Berg and A. H. Kerstjens, Rec. trav. chim., 1916, 35, 320-345. 
20 J. Duclaux and E. Wollmann, Bull. Soc. chim., 1920, 27,414. 21 R. O. 
Herzog and W. Jancke, Ber., 1920, 588, 2162-2164; Z. Phys., 1920, 3, 
196-198; Z. angew. Chem., 1921, 34, 385-387. 


Additional References. 


C. F. Cross and E. J. Bevan, ‘‘ Cellulose’; ‘‘ Researches on Cellulose,”’ 
1895-1900, 1900-1905, 1905-1910. C.F. Cross and C. Dorée, ibid., 1910-1921. 
A. J. Hall, ‘* Cotton Cellulose.”’ 


CHAPTER, III 


NITRATION OF CELLULOSE 


Reaction between Cellulose and Mixed Acids—Specification of Acids—Acid 
Balance—Conditions of WNitration influencing Solubility—Ratio of 
Acid to Cellulose—Composition of Acid Bath—Time and Temperature— 
Preliminary Treatment of Cotton—Necessity for Strict Control— 
Varieties of Cotton Used—Cotton Specifications—Degree of Nitration— 
Nomenclature of Cellulose Nitrates—Nitration Plant—Direct Dipping— 
Centrifugal Nitration—Displacement Process—Nitration of Linters 
in United States—Removal of Acid—Boiling—Poaching—Bleaching— 
Drying of Nitrocellulose—Dehydration of Nitrocellulose—Properties 
and Specifications of Nitrocellulose. 

CELLULOSE may be nitrated by nitric acid alone, but in practice 

a mixture of sulphuric and nitric acid is always used. The process 

consists essentially in bringing cotton cellulose into contact with 

the mixed acid, and, after nitration is complete, washing out the 
acid. The cotton does not alter in appearance during the process, 
but it gains considerably in weight and becomes harsher to the 
touch and more resistant to water. It does not dissolve in the 
acids, so that the rate of nitration is determined by the amount 
of surface exposed to the acid, and the rate of diffusion of the acid 
mixture through the cotton hairs. The nitration process takes 


place according to the typical equation 
R°OH -- OH'NO, = R:0:NO, + H,0, 


but this does not express the whole of the conditions. The sulphuric 
acid plays a much more active part than merely to absorb the water 
formed during nitration. It certainly reacts with the cellulose, 
since combined sulphuric acid is always found in the product, and it 
may be that the nitration process consists partly of a replacement of 
combined sulphuric acid by combined nitric acid. One of the 
effects of the sulphuric acid is to act hydrolytically by opening up 
fresh hydroxyl groups, so that there is always a partial depolymerisa- — 
tion during nitration. 


Specification of the Acids. 


The acids used are of the usual commercial quality, and specifica- 
tions usually quote the percentage strength and the specific gravity. 
The nitric acid should not contain too much nitrous acid, and both 
acids should be reasonably free from suspended matter and dissolved 
solids. In practice they cannot be obtained entirely free from 
dissolved and suspended compounds, chiefly of iron and lead, 
derived from the materials used in the plant in which they are 


made. 
30 


Nitration of Cellulose 31 


Acid Balance. 


The nitration of cellulose never exhausts the acid bath of its 
nitric acid, so that the spent acid recovered after the nitration can 
be “ revivified ” by the addition of stronger acid and brought up 
to its original strength ready for use again. 

For example, in the preparation of cellulose nitrate of high 
solubility in ether—alcohol, 140 tons of cellulose nitrate was prepared 
from 2795 tons of an acid having the following composition :— 


H,S O, ° . . . 6 I . 7 % 
HNO, . . ° ° 23: aq % 
H,O . ’ . . 14:6% 
The spent acid recovered from the nitration contained :— 
HSO, . : 60-:9% 
HNO, ° : . . 20-2% 
H,O . ° ° . 1 8: 9 wt 
The revivifying acids available were mixed acid containing :— 
H,SO, . . . . 60:5% 
HNO, .. “atid . 31-:8% 
H,O . ’ . ° 7: 7 we 
and concentrated sulphuric acid containing :— . 
H,SO, . x 92% 
H,O : ' 8% 


To produce the original weight of nitrating bath of the same com- 
position it is necessary to mix :— 


Containing tons 


Acid. Weight. 
oo a. ae 1717 tons 
Revivifying acids : 
8) Ce eae 994 ,, 
(6) Sulphurie acid ...... 84, 
Ce ae een 2795 tons 
Percentages 





The subject of acid balance is a very important one, and has 
been dealt with exhaustively in the Technical Records of the 
Ministry of Munitions,! from which the example given above is 
taken. 


82 Cellulose Ester Varnishes 


Conditions of Nitration influencing Solubility. 


If potassium nitrate is to be made by treating potassium 
hydroxide with nitric acid, a small excess or deficit of nitric acid is 
not, speaking qualitatively, a serious matter, since it is easy to 
obtain pure potassium nitrate from the product. The nitration 
of cellulose is a much more difficult matter. Cellulose nitrate is not 
a definite chemical individual, and the composition and properties 
of the product of nitration depend on a number of factors: (a) the 
ratio of acid to cellulose, (b) the composition of the acid bath, (c) the 
time and (d) the temperature of nitration. To these factors may be 
added a fifth, (e) the physical state of the cellulose before nitration, 
for neither is cellulose a definite chemical individual. A variation 
in any one of these conditions leaves its permanent impress on the 
properties of the cellulose nitrate produced. 

(a) Ratio of Acid to Cellulose—Cotton is a bulky material, and 
in order to make it as susceptible as possible to the action of acids, 
it is always nitrated in a very open condition. A large quantity of 
acid is therefore necessary if only to bring all the cotton in contact 
with acid, and the ratio of acid to cotton in practice varies from 
about 30/1 to 50/1. Since nitration only takes place in com- 
paratively strong acid mixtures, a considerable excess of acid is 
always used, and the cellulose nitrate comes into some kind of 
equilibrium with the spent acid. It follows that if a still higher 
ratio of acid to cellulose is used, the final cellulose nitrate will be 
in equilibrium with a stronger spent acid and will therefore be more 
highly nitrated. The literature of nitration would have been much 
simplified if it had always been recognised that the degree of nitration 
depends on the final equilibrium and not on the original strength 
of the nitrating bath. 

(b) Composition of Acid Bath—Numbers of researches have 
been carried out to trace the influence of the composition of the 
acid bath on the properties of the cellulose nitrate produced 7°. 
The percentage of nitrogen increases with the strength of the acid 
bath, 7.e. roughly speaking, as the proportion of water in the acid 
bath rises, the percentage of nitrogen taken up by the cellulose falls. 
In all acid baths used on the large scale, the percentage of sulphuric 
acid exceeds that of nitric acid, and under these conditions the 
viscosity of the cellulose nitrate increases with increasing nitric 
acid content in the bath. The composition of a typical bath for 
producing cellulose nitrate with high solubility in ether-alcohol has 
already been given. 


Nitration of Cellulose 33 


_ (ce) and (d) Time and Temperature.—These factors are conveni- 
ently considered together. The time of nitration must not be 
unduly curtailed, as time is necessary for the complete penetration 
of the cotton hairs by the acid mixture. As far as the combination 
with nitric acid is concerned, as shown by determination of nitrogen 
percentage, there is little to be gained by prolonging nitration 
beyond half an hour. Extra time spent in nitration naturally 
diminishes the output of the plant, but it also reduces the viscosity, 
and modifies the solubility relations of the product, so that the 
time of nitration is not fixed by considerations of output alone. 

A high temperature also reduces viscosity, but a practical limit 
is placed to the employment of this factor by the tendency of the 
cellulose to oxidise if temperature is too high. This occurrence is 
known as “‘ fuming off’ and leads to the total destruction of the 
cellulose, and the loss of a considerable quantity of nitric acid in 
the form of oxides of nitrogen. 

(e) Cellulose Purification.—The purification of cotton for nitra- 
tion consists chiefly in treatment with hot alkali solutions in closed 
vessels. Sometimes a bleach is also given. Either treatment if 
- carried too far will begin to break down the cellulose structure and 
reduce its viscosity. Cellulose from wood yields in general less 
viscous esters than those from cotton. 


Need for Strict Control. 


It will be evident from the preceding paragraphs that in order 
to manufacture cellulose nitrate of uniform quality, strict control 
must be exercised over the properties of the original cellulose, the 
composition of the acid bath and all the conditions of nitration. 
Even when all this has been done, small variations will occur, 
perhaps the most difficult factor to control being the temperature 
in the interior of masses of reacting cellulose. These variations 
must be averaged by careful blending at a later stage of the 

manufacture. 


Cellulose for Nitration Purposes. 


In the early days of the manufacture of cellulose nitrate for 
explosive purposes, several disasters occurred owing to the insuffi- 
cient stability of the product. One explanation advanced to account 
for these explosions was the presence in the nitrated cotton of 
unstable nitro-derivatives of impurities in the original cotton. To 
diminish the danger from this source, cotton in the purest available 
form, namely, hanks of yarn, was adopted as the raw material. 
Researches by Abel? in the ’sixties and seventies proved. that if 

3 : . 


34 Cellulose Ester Varnishes 


the nitrated cotton were pulped, the purification effected by the 
washing process was greatly improved, and with the introduction 
of this process it was found possible to employ cotton waste from 
the spinning mills in place of the expensive hanks of yarn. The 
utilisation of cotton waste from the spinning mills is an industry 
in itself, and new methods of treating it were and are continually 
being discovered.’ Hence the cellulose nitrate industry has always 
had to contend with difficulties arising from variations in the quality 
of its principal raw material. Cellulose nitrate for explosive 
purposes, when the very highest stability is required, is made from 
sliver (waste made at the card, comber and draw frames) and from 
cop bottoms (the last portion of spun thread remaining on the 
spindles). For the preparation of varnishes, linters (the short hairs 
remaining attached to the cotton seed after the long hairs have been 
removed) are largely used, especially in the United States.? Lastly, 
there are the mixed waste swept up in the cotton mills, which 
contains miscellaneous dirt and oil, and hull fibre,!° which consists 
of the short hairs left on the seed after removal of the linters, and is 
obtained from the seed by the processes known as decortication 
and defibrination. All these materials require preparation before 
they can be used for nitration, and the greater the impurity the more 
drastic the treatment must be. In general, it consists of an alkali 
boil under pressure, thorough washing with water, sometimes a 
bleach, followed by acidification and washing again. 

Important factory investigations on the preparation of cotton 
for nitration were carried out during the war, and have been 
described by Punter.!! It was found that treatment with dilute 
alkalies under pressure tended to bring cellulose, derived from 
several different sources, down to an approximately constant entity 
yielding solutions of approximately uniform viscosity. By applying 
the results obtained to the treatment of cotton supplied to explo- 
sives factories during the war, it was possible to obtain a much 
more uniform output from the cordite machines, and to economise 
in the consumption of the ether—alcohol used as solvent. 

In the celluloid industry, cotton cellulose is frequently nitrated 
in the form of a uniform cotton tissue paper, which is capable of 
undergoing a very uniform nitration owing to the rapid and thorough 
manner in which it comes into contact with the nitrating acids. 

Wood cellulose has been nitrated for explosives purposes in 
Germany. It does not give a nitrate of so good a colour as cotton 
cellulose, but it may find application in the varnish industry. It 
is usually nitrated in the form of tissue. 


Nitration of Cellulose 35 


Cotton Specifications. 


A typical specification may be quoted from Piest, Stich and 
Vieweg.4* The cotton must be clean and white. It must not con- 
tain impurities such as sand, paper, untorn threads, knots or seed. 
It must be very loose and suitable for nitration without further 
treatment. It should not contain much dust, and the fibre must 
be long and not overbleached. Moisture must not exceed 9%, 
fat content 0-5%, ash 1% and wood gum 2%. It must not contain 
more than traces of chlorine. One gramme of the cotton pressed 
lightly by hand must sink in water within three minutes. 

Schwarz ® gives the following specification: Maximum ash 
0-6%, fat 0-4%, water 6%. A white colour, freedom from chlorine, 
acidity, dust and vegetable impurities. | 

In considering these and similar specifications it must be remem- 
bered that cellulose is not a chemical individual, but a residue which 
has resisted certain chemical treatments in the course of its 
manufacture (7.e., alkaline boiling, acidification and possibly bleach- 

ing). It is required for its properties as a colloid. It would be 
useless to specify that it must agree with a certain elementary 
analysis, since that would give no information about its suitability 
for the manufacture of esters possessing the required physical 
properties. The specifications given deal only with possible 
impurities, for the following reasons :— 

Moisture.—Cellulose is hygroscopic and absorbent, and a limit 
to the moisture content must be fixed for commercial reasons. 
Moreover, a high avidity for atmospheric moisture often denotes 
degradation of structure resulting from too drastic purification, 
while the nitration of damp cellulose increases the evolution of heat 
and the risk of fuming-off. 

Ash.—The mineral matter in cellulose is partly derived from 
the original inorganic content of the cotton hairs, partly from the 
water and alkali used in the preliminary treatment, and partly from 
adventitious impurity. The limit imposed is a safeguard against 
artificially weighted cellulose and faulty pre-treatment. 

Fat.—This is usually termed, more correctly, ether extract. 
Greasy, oily and resinous impurities may be derived from the 
original cotton or from external contamination. If from the former, 
the hot alkaline treatment of the cellulose has been insufficient. 
These impurities hinder the wetting of the cellulose by the acid, 

~ and may leave unstable nitro-bodies in the product. The time of 


36 Cellulose Ester Varnishes 


sinking in water is a rough but useful factory test for the same 
impurity. . 

Additional tests frequently agreed between buyers and sellers 
are the copper number,!5 staining tests,1° e.g., with a solution of 
methylene-blue, and determinations of loss of weight on boiling 
with alkali under specified conditions. The exact significance of 
some of these tests is not yet completely understood, but they 
indicate the extent to which reducing and acidic groups have been 
produced in the cellulose structure, and they can frequently be 
correlated with the behaviour of the cellulose after its conversion 
into nitrate. 

The most valuable test of cellulose for the varnish manufacturer 
is undoubtedly its viscosity. Cellulose is dissolved by cupram- 
monium solution, and a method has been worked out by Gibson, 
Spencer and McCall 1’ for standardising the procedure, and obtaining 
comparisons of the viscosity of cellulose from different sources. They 
also showed that the ‘viscosity of esters prepared from cellulose 
under controlled conditions is governed by that of the original 
cellulose. Varnish manufacturers would probably prefer in most 
instances to carry out viscosity determinations on the ester instead 
of the cellulose, since the procedure is simpler and the results can 
be directly translated into factory practice. Nevertheless, occasions 
may arise when the application of a direct viscosity test to the 
cellulose may be of great value. 


Degree of Nitration and Nomenclature. 


It has already been pointed out that although theoretically 
cellulose should be capable of nitration in three distinct stages 
corresponding to the mono-, di- and tri-nitrate, in practice it is found 
from the determination of nitrogen percentage that the composition 
rarely accords with any one of these stages. It is customary 
therefore to classify cellulose nitrates according to the amount of 
nitrogen which they contain, and the terms mono-nitrate, etc., have 
been largely abandoned. The classification by nitrogen percentage 
has one great advantage in that it indicates roughly the degree of 
inflammability of the material, those with high nitrogen percentages, 
e.g.. from 12:4 to 13:0%, being the varieties used as explosives. 
Its disadvantage lies in the fact that it tells nothing of the other 
properties of the material. Hence a rough and exceedingly arbitrary 
nomenclature has come into existence in the industry, and it must be 
emphasised that it rests on no scientific basis and is characterised 


Nitration of Cellulose 37 


by no sharp limits. The range of commercial cellulose nitrates is 
as follows :— } 


Nitrogen 

Percentage. Names and Uses. 

10-2-11-2% Sometimes called xyloidin in England. Pyroxylin in U.S. 
Characterised by solubility in camphor-alcohol. Some of 
the new low-viscosity nitrates said to have nitrogen as 
low as 11:0%. 

1]-2-12-4% Collodion cotton, nitro-cotton, pyroxylin. Chiefly used for 
lacquers and enamels. High solubility in ether—alcohol. 
Cellulose nitrate used in service cordite during latter part 
of war had about 12-2—12-4% nitrogen. 

12-4-13:0% | Guncotton. Characterised by diminishing solubility in 


ether—alcohol as nitrogen content rises, but still soluble 
in acetone. 





Nitration Plant. 


The conditions to be satisfied as far as practicable in any plant 
for the treatment of cellulose with nitrating acid are as follows :-— 


(1) It should ensure thorough contact between the cellulose 
and the acid. 

(2) It should be adapted to maintain the acid balance 
between spent and revivified acid, with minimum loss of acid. 

(3) Fumes should be minimised, with the object of diminish- 
ing the discomfort to the workers, reducing losses of acid, and 
avoiding the corrosive effect of the fumes on other plant and 
fittings. 

(4) Output should be high, so as to reduce labour charges. 

(5) Repairs should be low. 


Obviously, in all nitration systems, a compromise has to be 
made between these conditions when they come into conflict. 

For a detailed account of processes of nitration the literature 
must be consulted. A good description is given by Nathan.1® The 
methods may be roughly classified as follows :— 


(1) Nitration in pans by direct dipping of the cotton in the 
acid. | 
(2) Nitration in centrifugals. 
(3) Nitration by displacement process. 


(1) Nitration by direct dipping is historically the oldest process. 
A weighed quantity of cotton is immersed portion by portion in a 


38 Cellulose Ester Varnishes 


measured quantity of the mixed acid. Special precautions are taken 
to withdraw fumes from the workers, who wear protective clothing 
and sometimes hoods supplied with compressed air. The bulk 
of the acid may be removed by hydraulic pressure, but more 
frequently by spinning in a centrifugal machine. The resulting 
mass is then quickly drowned by sudden immersion in a large 
quantity of water. 

The chief disadvantages of this method are low output, occasional 
fuming-off, a great deal of acid fume to be dealt with by ventila- 
tion and somewhat heavy repair and replacement charges. 

(2) A logical development from the last system is nitration in 
the centrifugal itself, so as to avoid the transfer of the cotton and 
acid from the dipping pans to the centrifugal. This process was 
developed chiefly in Germany. It gives a high output per man, 
and therefore a low labour charge. The capital cost of the plant 
is high, and repairs somewhat heavy, as the machines are working 
in a very corrosive atmosphere. 

(3) The displacement process was developed very considerably 
during the war to supply nitro-cotton soluble in ether—alcohol for 
cordite manufacture. The fundamental difference between this 
process and those previously described lies in the manner in which 
the cotton is freed from acid after nitration. Instead of a sudden 
drowning of the mixture of acid and cotton in a large volume of 
water, an extremely slow displacement is effected, by allowing water 
to flow in slowly at the top of the nitrating pan while the spent acid 
is withdrawn at the same rate below. ‘The aim in each process is 
the same, namely, to remove the acid from the cotton without the 
great rise in temperature which occurs when a strong nitrating 
acid is mixed with a moderate quantity of water. The displace- 
ment process is carried out in wide, shallow earthenware pans. 
The measured charge of acid is first introduced, and the cotton in 
as bulky and open a condition as possible is fed in slowly by hand 
and pushed under the surface with a fork. While this operation 
is proceeding, the pan is covered with an aluminium hood con- 
nected to a fume draught. After the cotton has all been introduced 
and packed as evenly as possible, perforated earthenware plates in 
segments are carefully placed on top of the mass so as to cover the 
whole surface and a small quantity of cold water is allowed to run 
gently over the plates. If the ratio of acid to cotton, the original 
mechanical preparation of the cotton, and the packing of the pan are 
correctly judged, the acid should meet the water layer at a level 
between the upper and lower surfaces of the earthenware covers, 


ig Nitration of Cellulose 39 


7.€., in the perforations, so that the actual surface of contact of 
the water and acid is at a minimum. As soon as the film of water 
covers the plates, all fuming from the acid is prevented, and the 
hoods may be removed, ‘The time of nitration may vary from one 
to four hours, according to the properties desired in the product. 
When nitration is completed, acid is slowly drawn off at the bottom 
of the pan and water is run in at the same rate above. The density 
of the acid is 1-6 to 1-7, so that there is little tendency for the acid 
and water to mix, and the acid is slowly displaced by the water. 
The composition of the spent acid issuing below is checked by means 
of a hydrometer, the readings of which remain practically constant 
until the zone of acid and water, which results from a slight mixing 
of the layers, reaches the discharge pipe. This zone can also be 
recognised by its higher temperature. The strong spent acid is 
collected in one set of receivers, and a proportion of the weak spent 
acidin another. The point at which the collection of weak spent acid 
is stopped is fixed by the economical consideration of whether it pays 
to recover the acid. 

The following advantages are claimed for the displacement 
process :— 

(1) Economy of Acid.—The recovery of acid is more complete 
than is obtained by the use of centrifugal wringing or by hydraulic 
pressure. It is usual to revivify the greater portion of this, and to 
return the rest to the acid plant to be treated for the recovery of 
the nitric and sulphuric acid which it contains. 

(2) Output.—This naturally depends on the time of nitration, 
but it compares favourably with the output of the other processes 
(except the centrifugal process) making the same product. The 
following figures are given by Nathan :— 7 


Output per Man per Week. 


Abel process (Waltham Abbey) 458 lb. 

Direct dipping (Ardeer) . ; 1112 Ib. 

Nitrating centrifugal : j (3000) lb. (Marshall) 
Displacement (Waltham Abbey) 1742 lb. 


The number of hours worked per week is not stated, but is presum- 
ably the same for each process. 

(3) There is no handling of acid nitro-cotton. The acid is 
removed in the original nitrating vessel and the final product is wet 
with water only. 

(4) The loss of nitro-cotton and acids by fuming-off is much 
diminished. 


40 Cellulose Ester Varnishes 


(5) There are savings in power, water and repairs. On the 
other hand, the initial capital cost of the plant is high, and the full 
advantage of acid economy cannot be realised unless the process is 
used in conjunction with plant for treating the excess of waste 
acid. 

Kirkpatrick has given an interesting description of the purifica- 
tion and nitration of linters for lacquer manufacture. The bales of 
linters are opened, broken up, teased and freed from dust. The 
cotton is then blown into autoclaves, in which it is boiled with a 
dilute alkali solution, the final conditions being a two-hour boil at 
100—110 lb. pressure maintained with live steam. The product 
is thoroughly washed, then bleached under carefully controlled con- 
ditions, acidified and washed again: The purified cotton is dried 
down to a moisture content of 14°%,and in this condition it is nitrated. 
The nitration plant is a development of the direct dipping method. 
The nitrating pans are steel drums with conical bottoms, 24 ft. 
in diameter and 4ft.in depth. Each is stirred by revolving paddles. 
35 lb. of cotton are nitrated with 1500 lb. of mixed acid for 25 
to 30 minutes. The charge is then tipped into a centrifugal machine 
to remove the bulk of the spent acid, which is collected and revivified 
for future use. The remainder of the acid is removed in a drowning 
vat. 


Removal of Acid from Nuitro-cellulose. 


The stability of nitrocellulose depends almost entirely on the 
completeness with which it is purified after nitration. There are 
two sources of acidity which may give trouble, (a) acid held mechanic- 
ally in the cotton; (b) sulphuric acid chemically combined with the 
cellulose as a sulphuric acid ester. The former may be entirely 
removed by repeated washing with cold water, preferably a hard 
water containing calcium and magnesium bicarbonates. The 
removal of free acid from the interior of the fibres is a diffusion 
process involving time, and the washing must not therefore be unduly 
hurried, even if the expenditure of water remains the same. For 
many purposes, a nitro-cotton so purified is sufficiently stable, 
particularly if it is to be used in enamels containing basic pigments 
such as zinc oxide. If, however, the highest stability is required, 
it is necessary to remove the greater part of the combined 
sulphuric acid also. The combination between the sulphuric acid 
radical and cellulose is not a stable one, particularly in the presence 
of traces of moisture and in a warm atmosphere. Free sulphuric 
acid is slowly liberated, and this has a hydrolysing action on the 


Nitration of Cellulose 41 


nitric ester, liberating nitric acid and weakening the film. The 
removal of combined sulphuric acid is carried out by boiling with 
several changes of water. The combined nitric acid is scarcely 
affected by this process of the reaction if the water is kept slightly 
acid. The boiling process has long been used in the stabilisation 
of gun-cotton,!* but was first put on a scientific basis by the 
researches of Robertson.® It may require local modifications 
depending on the hardness of the water supply and the amount of 
combined sulphuric acid to be removed, the latter depending again 
on the kind of cellulose nitrated and the composition of the acid- 
bath. 

The boiling process is followed by a cold washing process to 
ensure the complete neutralisation of the liberated acid. It should 
be noted that the boiling process has a marked influence on the 
solubility relations of nitro-cotton and tends to make it less viscous. 

Washing is carried out in vats resembling the poachers used 
by paper-makers. These are provided with a revolving drum 
furnished with knives bearing against similar knives on a bed- 
plate, so that the nitrocellulose is circulated, and at the same time 
cut up. A poacher may deal with a large charge of nitro-cotton— 
five hundredweight or more—and therefore effects a blend of a 
number of separate nitrations. For varnish manufacture this blend- 
ing is sufficient. In the manufacture of explosives, blending must 
be carried further, since the uniformity of explosive power depends 
on keeping the percentage of nitrogen constant. 

Bleaching.—For most purposes it is unnecessary to bleach the 
nitrocellulose, but if a nearly colourless lacquer is required, it is 
advantageous to bleach with bleaching powder solution in a faintly 
acid solution. The excess of bleach is usually removed with sulphur- 
ous acid, followed by a thorough washing. 


Removal of Water: Drying and Dehydration. 


The bulk of the water in the wet cotton so obtained is wrung out 
in simple centrifugal machines, yielding a product containing 
usually from 35—40% of water. If the nitro-cotton is to be trans- 
ported to a distance before use, it is usual to pack it in this form, 
which is recognised by the railway companies as safe. 

The complete drying of the nitro-cotton is a somewhat dangerous 
process. It is carried out in stoves heated to a temperature of about 
40° C., and ventilated to carry off the moisture-laden air. Factories 
carrying on this process come under the provisions of the Explosives 
Acts, and the most stringent safety precautions must be taken. 


4.2 Cellulose Ester Varnishes 


Fortunately for the cellulose nitrate varnish industry, alcohol 
is a constituent of all the ordinary varnishes, and nitro-cotton may — 
be dehydrated in safety by means of alcohol. The process is essen- 
tially the same as the displacement nitration process, since water is 
displaced from the wet cotton by the lighter liquid alcohol. There 
are several variants of the process. Nitro-cotton for cordite 
(R.D.B.) was dehydrated during the war in the Dupont press, by 
passing a stream of alcohol under pressure through a mass of damp 
nitrocellulose in a hydraulic press, the spent alcohol passing away 
through perforations in the face of the lower ram. This process is 
much quicker than displacement under atmospheric pressure, or 
under the usual low pressure of compressed air, and is economic 
in alcohol. Another method is by spraying with alcohol in a centri- 
fugal machine, but this needs a larger quantity of alcohol. In all 
of these processes, the spent alcohol is collected, and redistilled to 
bring it up to its original strength for re-use, and in passing it may 
be mentioned that this process may not be carried on except 
under the supervision of officers of the Board of Customs and 
Excise. 

The requirements of the varnish manufacturer in regard to 
dehydration are that the water shall have been effectively removed 
and replaced by alcohol of such a strength that the nitro-cotton 
will dissolve in the chosen solvents and film without blooming. 
The amount of alcohol present in the material must, of course, be 
taken into account when weighing the nitro-cotton and measuring 
the liquid ingredients of the varnish. 


Description of the Dehydrated Nitro-cotton. 


Nitro-cotton made by the above processes closely resembles 
the original cotton, but the fibre is shorter and feels harsher. It 
has a tendency to ball together into “ pills,’ which, however, are 
easily opened out again. The properties of chief interest to the 
manufacturer, and those which are usually incorporated in a 
specification, are the following :— 

(1) Solubility. —This is measured in a specified solvent or mixture 
of solvents. If, for example, butyl acetate solutions are of most 
interest to the manufacturer, he may specify a certain maximum 
quantity of insoluble matter in a solution in butyl acetate of given 
concentration. When the solvent specified has a high boiling point 
such as that of butyl acetate, the amount of insoluble matter is 
determined usually by centrifuging the solution for a certain time 


Nitration of Cellulose 43 


under defined conditions, and measuring the volume of insoluble 
matter thrown down to the bottom of the tube. If the solvent 
is readily volatile, a determination can easily be made of the actual 
amount of nitro-cotton in solution, by evaporating a measured 
volume to constant weight or by weighing the insoluble residue. 
The centrifugal method is quicker. 

(2) Viscosity —This property is measured at a standard con- 
centration in a specified liquid at a definite temperature, either by 


(a) measurement of the rate of flow in a capillary tube, or 
(b) by the rate of fall of a small steel ball in a column of 
the liquid. 


Method (a) is most suitable for a highly fluid solution (low 
viscosity), since the rate of flow of viscous solutions in a capillary 
tube is so slow that the measurements take too much time. 
Method (b), on the other hand, is difficult to apply with accuracy to 
a fluid solution, since the time of fall is too rapid to be measured 
with accuracy, but it is admirably adapted to viscous solutions. 
It is desirable that measurements of viscosity should be made in 
solutions approaching as nearly as is practicable the concentrations 
of the finished varnishes. Hence, in general, the falling sphere 
is to be preferred to the Ostwald instrument for technical purposes, 
and it is rather surprising that the British Engineering Standards 
Association should have adopted the Ostwald viscometer in its 
specification. 

In addition to the obvious limitations as to moisture content, 
ash and acidity, the following properties may also be specified. 

(3) Stability—Imperfectly purified nitro-cotton tends to become 
acid on keeping, and although in the varnish trade (unlike the explo- 
_ sives industry) this defect is not attended with danger, it is never- 
theless often desirable that a maximum rate of decomposition should 
be fixed. This is particularly so when the varnish films are required 
to withstand severe atmospheric conditions, but it is also advantage- 
ous when the varnish is to be used in contact with any metals, 
either as a protective lacquer, or as a medium for applying metal 
powder. 

Several methods of testing the stability of cellulose nitrate are 
known. Since even the most unstable product is very lasting at 
ordinary temperatures, the tests are always applied to the nitro- 
cotton heated to a somewhat high temperature, and the assumption 
is made that the stability at this high temperature affords an 
estimate of the stability, or the length of useful life, at ordinary 


4A Cellulose Ester Varnishes 


temperatures. The two commonest tests are the Abel heat test, 
and the Bergmann and Junk test. 

The Abel heat test depends on the fact that when cellulose 
nitrate is exposed to a temperature of 170° F. (76-7° C.) it is slowly 
decomposed and evolves nitrous fumes. The time is taken for these 
fumes to reach a concentration at which a piece of moistened filter- 
paper treated with starch and potassium iodide and suspended in 
the tube containing the sample, first begins to show discoloration. 
The significance of the heat test was discussed by Robertson and 
Smart in 1910,?° and the method of conducting the test is described 
in the ‘“‘ First Report of the Departmental Committee on the Heat 
Test as applied to Explosives ’’ (1914). 

The Bergmann and Junk test originated in 1904.21 In this 
test the nitro-cotton is heated to a much higher temperature (132° C.) 
for two hours, and the acid fumes evolved are collected in water and 
estimated. In the original form of the test, the amount of nitrogen 
in the water was determined by what is known as the Schultze- 
Tiemann method. The British Engineering Standards Association 
have adopted the test, but estimate the collected nitrogen acids by 
a simple acidimetric titration. 

(4) It has already been mentioned that the instability of nitro- 
cotton is greatly increased by the presence in it of combined 
sulphuric acid, and that the efficacy of the boiling process as a 
stabilising agent depends on the reduction in the percentage of this 
ingredient. Hence an additional but indirect stability test is made 
by determining the amount of sulphur in the nitro-cotton. This 
is carried out by oxidising a weighed quantity with nitric acid and 
potassium or sodium chlorate, and precipitating the sulphuric acid 
thus produced as barium sulphate. The B.E.8.A. limit for nitro- 
cellulose for aircraft dope is 0-1% (calculated as H,SO,). For 
varnish for ordinary purposes, the limit need not be so small, and 
often no limit is specified. 

(5) Although the nitrogen content of nitro-cotton has no bearing 
on its usefulness as a base for varnish, it is a useful check against 
irregularities in nitration and is so easily applied that it should not 
be omitted. The best method of determination is the measurement 
of the nitric oxide evolved when the sample is boiled with a 
solution of ferrous chloride in hydrochloric acid (Schultze-Tiemann). 
Perhaps the most widely-used method is that of Lunge, in which is 
measured the volume of nitric oxide evolved when the sample is 
shaken with mercury and sulphuric acid. Details of both methods 
are given in standard books on analysis. 


Nitration of Cellulose 45 


The B.E.8.A.” specification for nitro-cotton for aeroplane 
varnish is contained in No. 2, D. 8 of Nov. 1921, Par. 6 and appendix. 
This specification is more stringent than is necessary for ordinary 
varnish work, but strictness is necessary on account of the very severe 
conditions to which aeroplane varnishes are subjected. 


REFERENCES AND BIBLIOGRAPHY. 


1 “Tech. Records of Explosives Supply No. 4. Theory and Practice of 
Acid Mixing.”” # G. de Bruin, Rec. trav. chim., 1921, 40, 632. %° G. Lunge, 
J. Amer. Chem. Soc., 1901, 28, 257; Z. angew. Chem., 1906, 19, 2051-2058. 
4 C. Piest, ibid., 1909, 22, 1215-1224. 5 C. N. Hake and M. Bell, J. Soc. 
Chem. Ind., 1909, 28, 457-464. ® C. Piest, Z. angew. Chem., 1910, 23, 1009— 
1018. 7 F. Abel, Trans. Roy. Soc., 1867, 181-253. §® T. Thornley, ‘‘ Cotton 
Waste.” °* S. K. Kirkpatrick, Chem. and Met. Eng., 1924, 31, 178-182. 
10 #}. C. de Segundo, J. Roy. Soc. Arts, Feb. 5,1919. 11 R. A. Punter, J. 
Soc. Chem. Ind., 1920, 39, 3337. 1% C. Piest, E. Stich and W. Vieweg, ‘‘ Das 
Celluloid,” p. 17. 1% R. Schwarz, Oester. Chem. Zeit., 1919, 22, 50-52, 57— 
60. 15 D. A. Clibbens and A. Geake, J. Text. Inst., 1924, 115, 27. 18 C. 
Birtwell, D. A. Clibbens, and B. P. Ridge; ibid., 1923, 14, 2977. 17 W. H. 
Gibson, L. Spencer and R. McCall, Trans. Chem. Soc., 1920, 117, 479; see 
also R. Robertson, ‘‘ Aeron. Research Committee, Rep. and Memor.,”’ No. 734, 
Sept. 1920; and R. A. Joyner, Trans. Chem. Soc., 1922, 121, 1511, 2395. 
18 fF, L. Nathan, J. Soc. Chem. Ind., 1909, 28, 177-187. 19 R. Robertson, 
ibid., 1906, 25, 624. °° R. Robertson and B. J. Smart, zbid., 1910, 29, 130- 
138. 71 E. Bergmann and A. Junk, Z. angew. Chem., 1904, 9, 982, 1018 and 
1074. *2 To be obtained from the secretary, 28 Victoria Street, S.W., 
price 2d. post free. 


Additional References. 


A. Marshall, ‘‘ Explosives’? (1915), chaps. vii.xii. E. C. Worden, 
** Nitrocellulose Industry,’’ Vol. I., chaps. i.-iii. EE. C. Worden, ‘*‘ Technology 
of Cellulose Esters,” Vol. I. H. Barthélemy, Rev. des prod. chim., 1921, 
24, 301-306 (purification and bleaching of cotton linters). J. O. Small and 
C. A. Higgins, Chemical Age (New York), June 1920 (‘‘ Notes on the Manu- 
facture of Nitrocellulose,” with photographic illustrations of plant). W. 
MacNab, “‘ Lectures on Explosives,”’ Institute of Chemistry, 1914. M. A. W. 
Sapojenikoy, ‘‘ Theory of the Nitration of Cellulose,” Seventh International 
Congress of Applied Chemistry, Section IIIs, ‘ Explosives”’ (Partridge & 
Cooper, 1910). F.C. Axtell, J. Ind. Hing. Chem., 1913, 5, 38-47 (a description 
of the plant of the Sakai Celluloid Co., Japan). J. R. Dupont, Chem. Met. 
Hing., 1922, 26, 11-16 (‘‘ Manufacture of cellulose nitrate for pyroxylin 
plastics ’’). Most of these articles are accompanied by illustrations and 
drawings of plant. 


CHAPTER IV 
ACETYLATION OF CELLULOSE 


Reaction between Cellulose and Acetylating Bath—Differences from Nitra- 
tion Process—Catalysts—Non-inflammable Cinema Films—Low Viscosity 
of Acetates—War Research on Dopes—Principal Manufacturers and 
Outline of their Patented Processes—Investigations on Partial Hydra- 
tion—Acetylation Processes in which the Acetate does not Dissolve— 
Acetylation Plant—Properties and Specification of Cellulose Acetate. 

CELLULOSE acetate is produced when cellulose combines with 


acetic acid in accordance with the typical equation of acetylation :— 
R°OH + HOOC’CH, = R°0°CO’CH, + H,0. 

In order to promote the reaction, it is necessary for a so-called 
catalyst to be present, and the substance almost always used for 
this purpose is sulphuric acid. Nominally, its function is to com- 
bine with the water produced during the esterification. Actually, 
as in the parallel instance of nitration, it must play a more important 
part, and the final product always contains combined sulphuric 
acid. Further, cellulose cannot be acetylated by acetic acid alone. 
It is necessary to use the more powerful acetylating agent acetic 
anhydride. This substance alone is capable of taking up all the 
water produced in the reaction, which is additional evidence that 
the sulphuric acid is required for some other purpose. 


Differences from Nitration Process. 


Although nitration and acetylation are analogous in theory, 
in practice some important differences are found. The most obvious 
arises from the fact that cellulose acetate is soluble in the mixture 
of acetic acid, acetic anhydride and sulphuric acid used for the 
acetylation. Hence as the cellulose is acetylated it passes into 
solution, and the final product of the reaction is a viscous mass 
from which the ester is obtained by precipitation with water. In 
the next place, the time required for acetylation is much longer 
than the time required for nitration. Lastly, the composition 
of the acid bath is very different from that used for nitration, 
the percentage of sulphuric acid being very much smaller. 


Catalysts. 


Many substances have been investigated with the object of 
substituting them for sulphuric acid as the catalyst of the reaction. 
Sulphuric acid has a strong hydrolytic effect on cellulose, and owing 
to the slowness of the acetylation process, the cellulose always 
undergoes a considerable degradation in structure simultaneously 
with acetylation. One of the principal subjects of research in the 


manufacture has been to find a means of reducing this degradation 
46 


Acetylation of Cellulose Av 


to the smallest limits so as to produce a more viscous acetate. This 
consideration has not been of much importance in the manufacture 
of cellulose acetate for solutions, where low viscosity is, within 
limits, often advantageous, but it has been recognised that the tough- 
ness and strength of ordinary celluloid are closely related to the high 
viscosity of its solutions, and that cellulose acetate is not likely 
to replace cellulose nitrate in celluloid manufacture until it can 
be made to give solutions of equally high viscosity. 

It has been pointed out? that all substances which assist the 
reaction between cellulose and acetic anhydride are solvents of 
both components. Perhaps it would be more correct to say that 
the catalyst is capable of being adsorbed by the cellulose, and is 
also miscible with aceti¢ anhydride, so that the latter is brought 
into intimate contact with swollen cellulose, exposing a large sur- 
face to acetylation. There is probably also a secondary reaction 
involved in which the sulphuric acid ester of cellulose is converted 
into the acetic acid ester. The rate of formation of cellulose acetate 
is governed, not by the rate of acetylation of the hydroxyl groups 
of the cellulose, but by the rate of diffusion of the acetylating mixture 
into the cellulose, which is much slower. Many of the substitutes 
for sulphuric acid which have been patented are compounds of sul- 
phuric acid in which the hydrolytic action of the acid is presumed 
to be diminished, e.g., sodium bisulphate, sulphates of organic 
amines, metallic sulphates such as copper sulphate, chamber crystals 
(nitrosyl sulphuric acid), sulphonyl chloride. It may be doubted 
whether the inhibition of the hydrolysing power of the acid does 
not correspondingly retard acetylation, and the use of sulphuric 
acid in the manufacture on the commercial scale is believed to be 
almost universal. 

The brief sketch in the first chapter brought the history of 
cellulose acetate up as far as the important hydration process of 
G. W. Miles, by which it was proved that the solubility relations 
of cellulose acetate hitherto known could be entirely modified. 
In following the subject from this point to the present day it must 
be constantly borne in mind that the main objective of the investi- 
gators and manufacturers up till the outbreak of war was not the 
development of cellulose acetate varnishes. A much more valuable 
prize was waiting for the manufacturer who could produce from cellu- 
lose acetate a film comparable with the celluloid film which was 
beginning to be used in such enormous quantities for cinema 
projection. A more difficult problem still was the substitution of 
cellulose acetate for cellulose nitrate in the manufacture of celluloid 


. 


‘é 
48 Cellulose Ester Varnishes 


articles, and it may be remarked parenthetically that few of the 
patentees of the period seemed to know that celluloid cinemato- 
graphic films are made from an entirely different variety of cellulose 
nitrate from that used in celluloid articles; or, if they knew, they 
did not appreciate the importance of the fact, for one continually 
meets with the tacit assumption that a variety of cellulose acetate 
suitable for films would also be adapted to the manufacture of 
celluloid articles. 

The most evident difference between the early varieties of cellu- 
lose acetate and the cellulose nitrate used in cinema films lay in the 
lower viscosity of the former and the weakness and brittleness of 
the films made from it. Dreyfus is usually credited with having 
been the first to correlate these two facts, and to realise that in order 
to obtain a stronger film, a more viscous ester must be made, but 
the deduction was so obvious that it must have been made by many 
of those engaged in the celluloid film industry as soon as cellulose 
acetate appeared on the market. From about 1907, therefore, 
until 1914, the energies of chemists were bent on producing cellulose 
acetates of higher viscosity without sacrificing the wider range of 
solubility which the Miles process had made possible, 

After the outbreak of war, the direction of research was gradually 
altered. It has already been shown how cellulose acetate was 
needed for the doping of aeroplane wings, and how the manufacture 
of the ester for this purpose had to be increased enormously. The 
need for high viscosity was now not so great, as satisfactory dopes 
could be made from an ester of medium viscosity which would not be 
suitable for making a celluloid substitute. It is somewhat para- 
doxical that the cellulose nitrate manufacturers since the war should 
have directed their efforts to obtaining an ester of low viscosity, 
which the acetate manufacturers before the war were doing their 
best to avoid. | 

Information on the progress of the industry during these periods 
is almost entirely confined to the patent literature, which is unusually 
intricate and obscure, fully deserving the caustic comment of Briggs.? 
It will therefore be most informative to give the names of those 
firms who were actually manufacturing cellulose acetate in quantity 
at the outbreak of war, and to outline briefly the lines of advance 
indicated by their patents. According to Drinker,’ these were :— 


In France :—The Société Chimique des Usines du Rhone. 
In Germany :—The Bayer Company at Leverkusen. 
In Switzerland :—The Cellonite Company (Dreyfus Bros.) at Bale. 


Acetylation of Cellulose 49 


Worden ® mentions also that small amounts were being made by 
a firm at Lyons in France, and by another near Boston, Mass., 
for the manufacture of artificial silk. 

The Société Chimique des Usines du Rhone patented in 1911 ° 
an entirely novel procedure in acetylation, in which the vapour of 
acetic anhydride is the active agent. Since the boiling point of 
acetic anhydride at atmospheric pressure is too high, partial vacuum 
is employed so that the acetylation takes place at about 50—55°, 
The cellulose may be either dry, damp with water or with a dilute 
acid. The patent describes as an example the acetylation of cotton 
impregnated with sulphuric acid. This is placed in a vessel con- 
nected with a condenser, receiver and vacuum pump. The vessel 
is heated to 55°, while the pressure is reduced to 30 mm. The vessel 
is then connected with a heated still containing acetic anhydride, 
which boils under this pressure at 50—55°. The vapour of acetic 
anhydride passes through the mass of cotton and acetylates it, any 
excess of anhydride being condensed and collected. 

In 1913 the same company patented a process 7 on more con- 
ventional lines, in which the cellulose is given a preliminary treat- 
ment which recalls the earlier processes in which the cellulose is 
modified by treatment with mineral acids previous to acetylation. 
It differs from them in that acetic anhydride is present during the 
initial treatment. For example, 10 parts of cellulose is soaked 
for several hours at 30° in a mixture of 60 parts of glacial acetic acid, 
4 parts of acetic anhydride, and 0-5 part of sulphuric acid. Twenty- 
one parts of acetic anhydride is then added, and the cellulose is 
rapidly acetylated and dissolved. It is recovered by precipitation 
with water in the usual way and yields an ester soluble in chloroform, 
very slightly soluble in alcohol, and insoluble in nitrobenzene, 
acetone or ether. Partial hydrolysis modifies the solubility and may 
be made to yield esters soluble in acetone. 

In 1914,8 the Usines du Rhone patented a process of acetylation 
in the presence of trioxymethylene, the treatment with which may 
either precede or accompany acetylation. For example, 10 parts 
of cellulose is treated for some hours at 30° with 60 parts of glacial 
acetic acid, 4 parts of acetic anhydride, 0-5 part of 100% sulphuric 
acid and 1 part of trioxymethylene. Twenty-one parts of acetic 
anhydride is added, and acetylation is carried on at 40°. 

A variant of this process was patented later in the same year ® 
in which the trioxymethylene is first converted into methylene sul- 
phate, by treatment with fuming sulphuric acid, and the methylene 
ieumert is used as catalyst without the addition of free sulphuric 


50 Cellulose Ester Varnishes 


acid. In another patent in the same year,!° the process described 
in F.P, 473,399 (q.v.) is modified by the use of a little nitric acid 
in the preliminary treatment. The product of acetylation is described 
as a nitroacetate, and may contain, for example, 0:5°% of combined 
nitrogen. 

In 1920 another variation of the process was patented 11 in 
which the preliminary treatment of the cellulose before the actual 
acetylation is carried out at a lower temperature (25—30°) and with 
less sulphuric acid present (3—5°%). The esters thus obtained are 
insoluble in chloroform. If partially hydrolysed before precipita- 
tion, the ester may be obtained in a form insoluble in chloroform 
and soluble in acetone, or soluble in ethyl acetate. 

The Bayer Company first appear in the patent list in 1901. 
In their earliest patent 12 they describe the preparation of an ester 
soluble in alcohol, by acting on a hydrated cellulose with acetic 
anhydride, acetic acid and sulphuric acid. Later they bought Miles’s 
patent, covering the production of acetone-soluble cellulose acetate 
by the hydration process. Worden? describes an acetylation 
by this process as follows: 100 parts of dry cellulose is acetylated 
with a mixture of about 300 parts of acetic anhydride and 400 parts 
of glacial acetic acid with 5 to 9 parts of sulphuric acid as catalyst. 
The temperature is first kept at about 40°, but may later rise to 
50—60°. After from 36 to 40 hours, the cellulose should have passed 
entirely into solution and a sample on precipitation should be soluble 
in cold chloroform, but not in acetone. The partial hydrolysis 
which is characteristic of Miles’s process is then begun, by adding 
slowly, without interrupting the kneading process, a mixture of 60 
to 65 parts of water and 60 parts of acetic acid. The product of 
the original reaction approximates to a triacetate, 7.¢e., an ester con- 
taining three acetyl groups to each ©, unit. After the addition 
of water, test samples are taken at intervals. It is found that the 
solubility in chloroform gradually lessens, while that in acetone 
increases. The process is stopped when the solubility in acetone 
is complete. At this point, the solubility in chloroform has dimin- 
ished until the acetate merely swells in that liquid without dispersing. ~ 
This process of allowing the cellulose acetate to stand while the hydra- 
tion and solubility changes are proceeding is usually spoken of as 
ripening. It may need from 12 to 16 hours at a temperature of 40— 
50°, and during this time the mass is removed from the kneaders 
and allowed to ripen in separate vessels, which are not usually 
stirred. ‘The cellulose acetate so produced yields stronger films 
than those obtained from a solution of the original acetate in chloro- 


Acetylation of Cellulose 51 


form. It should be noted that if the hydration of the acetate is 
allowed to proceed beyond this stage, the product becomes unaffected 
by chloroform, and the acetone solution will bear the addition of 
more and more water without precipitation of the ester.® 

A variation of Miles’s process is described by Bayer in 1906, 
in which dry cellulose acetate is soaked in hydrochloric acid until 
it is soluble in acetone. 

The course of the hydration process has been investigated by 
Ost and the results were published in a series of papers in 1919.15 
It is interesting to note that the hydration process bears some 
resemblance to the stabilising of cellulose nitrate by boiling water, 
in that combined sulphuric acid is eliminated from the ester with 
proportionately little disturbance to the combined acetic acid. For 
example, in one of Ost’s experiments, a primary acetylation was 
allowed to “‘ripen’’ with the addition of a little water for four 
days, during which the combined sulphuric acid dropped from over 
3% to 0-17%, and the acetic acid from 57-4% to 50-9%. 

Ost classified the cellulose acetates as follows :— 


(1) The primary products of acetylation are acetates soluble 
in chloroform, preferably with the addition of a little alcohol. 
They dissolve in acetone, but yield unsatisfactory films which 
cannot be entirely redissolved. 

(2) The secondary acetates are derived from the primary 
acetates by mild hydrolysis (hydration). They are easily 
soluble in acetone and yield good films which can be entirely 
redissolved. 


He found that the limits of acetone ening lay between 50-9 
and 57-69% of combined acetic acid. 

Generalisations on the solubility of cellulose esters are always 
unsafe, as there are so many factors which can be varied in their 
preparation. The solubility relations of primary acetates prepared 
by the later Dreyfus processes do not agree with Ost’s conclusions, 
as they are insoluble in chloroform and can be redissolved in acetone 
indefinitely.1* Nevertheless, Ost’s work contains much useful 
information on the influence of various catalysts on the course of 
the acetylation process, and has thrown much light on an obscure 
and difficult subject. 

In 19101” F. Bayer and Co. describe an acetylation process 
in which aniline bisulphate is used as catalyst instead of sulphuric 
acid. This process falls into the category mentioned earlier in 
the chapter of those in which an attempt is made to depress the 


52 Cellulose Ester Varnishes 


hydrolytic activity of sulphuric acid by combining it with another 
substance—in this instance an aromatic base. 

The Dreyfus patents are the most important of all from the 
technical point of view. The first patent appeared in 1911,'® 
and it has been followed by a long series of patents right up to the 
present time, dealing with all aspects of the manufacture and utilisa- 
tion of cellulose acetates. A complete list is given by Worden.® 
The conditions to be controlled are (a) the ratio of the weights of 
acetylating agent and of condensing (or catalysing) agent to the weight 
of cellulose; (b) the ratio of the weight of condensing agent to the 
humidity of the cellulose, (c) the temperature. As an example, 
the directions in the French Patent 478,023 of H. Dreyfus 1° may be 
briefly abstracted: A mixture of 300—400 kilos. of glacial acetic 
acid, 250 kilos. of acetic anhydride and 10—15 kilos. of concentrated 
sulphuric acid is cooled to below 0°. One hundred kilos. of cotton 
or paper of normal humidity (3—6%) is stirred into the mixture. 
After a time the temperature rises to 5—15° and then falls again to 
5—10°. The cooling may then be discontinued, and the tempera- 
ture may be allowed to rise to 15—20°. If it is allowed to rise higher 
than 25—35°, the viscosity of the product will be diminished. 
Cooling is then started again with constant stirring until the tem- 
perature of the mass begins to fall, and the mixture is then allowed 
to stand until all the fibres have disappeared. At this stage water 
may be added and the mixture allowed to stand in order to modify 
the solubility of the product (cf. Miles’s process). This process is 
said to produce cellulose acetate of maximum viscosity insoluble in 
chloroform, and great stress is laid on the accurate observance of 
the limits of temperature allowed. 


Processes in which the Cellulose Acetate does not Dissolve. 


Mention should also be made of processes of acetylation in which 
the original form of the cellulose is preserved. This is achieved by 
diluting the acetylation mixture with a liquid in which cellulose 
acetate is insoluble, but which is miscible with the combined acetyl- 
ating reagents. The earliest patents on this subject are those of 
Lederer 2° and Mork.?!_ Lederer uses carbon tetrachloride as the 
non-solvent diluent, while Mork uses benzene. This process appears 
to have at least two great advantages over the usual process in 
which the cellulose acetate goes into solution. In the first place, 
the greater bulk of liquid and the greater ease with which the mass 
can be stirred must greatly facilitate temperature control. In 
the second place, the spent acetylating bath can be separated from 


Acetylation of Cellulose 538 


the acetate without having to use precipitation by water (which 
destroys any unused acetic anhydride and dilutes the acetic acid). 
One would expect, therefore, that it would be possible to revivify 
the spent bath in much the same way as a nitration bath is brought 
up to its original strength. The process has certainly been used 
on a commercial scale in the United States and the writer has examined 
cellulose acetate made in this way which showed excellent solubility. 
However, the method has not been widely adopted, and there is 
probably some technical disadvantage attached to it which does 
not appear on the surface. 

Very little information has been published about the plant used 
in the acetylation of cellulose, nor is it possible to tell from the patent 
literature what are the processes actually used in the factories. The 
outline is as follows : The kneading of the cellulose with the acetyl- 
ating mixture of acetic anhydride, acetic acid and sulphuric acid 
takes place in bronze-lined kneaders fitted with two shafts bearing 
curved blades, designed to shear the contents and keep them con- 
tinuously in motion. The kneaders are jacketed so that either 
warm or cold water, or refrigerating brine, may be circulated to 
control the temperature. Although the Dreyfus patents lay great 
stress on keeping the temperature below certain limits when working 
for cellulose acetate of high viscosity, this condition is not so impor- 
tant when the acetate is required for the manufacture of dope, and 
it will be noticed that the Rhone processes do not employ brine: 
refrigeration. If cold acetylation is required, the acetylating mix- 
ture is given a preliminary cooling in a copper or bronze vessel 
provided with coils for the circulation of brine, before it is introduced 
into the kneader. 

The Dreyfus factories are said to employ cellulose in the form 
of a special cotton tissue paper, and the Usines du Rhone a special 
long staple cotton. The British Engineering Standards Associa- 
tion 2? specify for cellulose used in the manufacture of cellulose 
acetate that it must be neutral in reaction and free from starch. It 
must not contain more than 8% of moisture, 0-5% soluble in ether, 
0-5% soluble in water, and it must not yield more than 0-5% of ash. 
When boiled with 3% caustic soda under specified conditions it 
must not lose more than 4% in weight. 

After the cellulose has dissolved in the acetylating mixture, it 
undergoes a ripening process, and for this purpose it is transferred 
to copper or enamelled vessels which are maintained in a room kept 
at the appropriate temperature, usually from 30° to 45°. When 
the examination of samples shows that the right degree of solubility 


54 Cellulose Ester Varnishes 


has been attained (which usually means complete solubility in 
anhydrous acetone), the cellulose acetate is precipitated by pouring 
it into a large volume of water. This is done in a wooden vat fitted 
with stirring gear. The aim of this process is to produce a precipi- 
tate in as fine a form as possible, so as to facilitate the washing-out 
of all traces of free acid, and it is difficult to control the conditions 
so as to avoid the formation of large clots. The liquor now con- 
tains the excess of acetic anhydride and acetic acid in the form 
of dilute acetic acid together with a small percentage of sulphuric 
acid. This is treated for the recovery of acetic acid, usually as 
sodium acetate, which can be employed for the manufacture of — 
acetic anhydride or glacial acetic acid. The precipitate of cellulose 
acetate is thoroughly washed, with a preliminary grinding if there 
are large lumps in it. It is then separated off by filtration or cen- 
trifuging, dried in warm stoves at 40—50° and ground up. It is 
particularly necessary that the ester should be in a fine condition 
so as to dissolve more readily. 


Properties of Cellulose Acetate. 


Cellulose acetate as it usually occurs in commerce is a white 
and somewhat dusty powder, not entirely free from small, tough 
lumps which were originally clots formed in the precipitation 
by water. The product of those processes in which the cellulose 
is not allowed to dissolve (Mork and Lederer) retains the original 
form of the cotton. Cellulose acetate, like cellulose nitrate, is much 
more resistant to water than the cellulose from which it is made. 

It can be distinguished from cellulose nitrate by its solubility 
in a mixture of chloroform and alcohol, or of tetrachlorethane and 
alcohol, and by the slowness with which it burns when heated on 
porcelain or platinum. 

Several methods have been proposed for the estimation of com- 
bined acetic acid, of which the simplest and best is probably that of 
Kberstadt,?? although Fenton and Berry *4 prefer Ost’s method.1® 
Both depend on the saponification of a weighed quantity of cellulose 
acetate by alkali. 

Kberstadt’s 25 method depends on the fact that cellulose acetate 
swells in a mixture of alcohol and water, and in the swollen state 
is saponified rapidly and completely by caustic alkali. 0-2—0-3 
gramme of cellulose acetate is moistened with a small quantity 
of alcohol, to which 10 c.c. of N-alkali is added. The flask is 
shaken occasionally, and at the end of an hour the excess of alkali 
is titrated with normal acid.*¢ 


Acetylation of Cellulose 55 


Alkaline saponifications are preferred by most authorities to 
the acid methods in which the acetate is hydrolysed by sulphuric 
or some other non-volatile acid, and the acetic acid distilled off in 
steam. 

Barnett 1 describes another variation of the alkaline method 
in which the sample is dissolved in acetone, and saponified by shaking 
it with standard caustic soda in a stoppered flask. He indicates 
a slight correction due to neutralisation of soda by the cellulose. 

Barr and Bircumshaw 2’ prefer Barthélemy’s 28 method of hot 
saponification with normal aqueous sodium hydroxide at 85°, 
stating that they have obtained more concordant results by this 
method than by older ones, although the results are not necessarily 
more accurate. 


Determination of Sulphur. 


The presence of combined sulphuric acid in cellulose acetate 
has the same detrimental influence on the stability of the ester as 
it has on cellulose nitrate. It may be determined by oxidising a 
weighed quantity with aqua regia, or with a mixture of nitric acid 
and potassium chlorate, and precipitating the barium chloride as 
sulphate with the usual precautions. 

Entat and Vulquin ?® hydrolyse with water under pressure at 
125°, and estimate the sulphuric acid by electrometric titration. 
They state that the percentage of combined sulphuric acid in a 
good sample should not exceed 0-:03°%. This is a somewhat stringent 
limit, as a good sample analysed by Barr and Bircumshaw (loc. cit.) 
contained 0:04%. 


Charring Test. 


A second useful stability test developed during the war was the 
determination of the temperature at which charring is first noticed. 
The charring is chiefly due to the liberation and concentration of 
liberated traces of sulphuric acid. Barr and Bircumshaw 2? mention 
the test and it has been standardised by the British Engineering 
Standards Association. 

A third stability test measures the loss of acetic acid when the 
sample is heated to a high temperature for a fixed time (compare 
the Bergmann-Junk test for nitrocellulose). In its earlier form,?’ 
this was carried out by passing air through the acetate and collecting 
in standard alkali the acetic acid vapour carried off. In its standard- 
ised form the sample is heated with water in a sealed tube for a 
specified time and the acid set free is titrated with standard alkali. 


56 Cellulose Ester Varnishes 


Solubility is determined in a chosen solvent by dissolving the 
acetate in the ingredients of the solvent under specified conditions, 
spinning the solution in a centrifuge and reading off the volume 
of insoluble matter. If volatile solvents are chosen, such as acetone 
or methyl acetate, the weight of ester in an aliquot portion drawn 
off from the clear layer after the solution has stood for some hours, 
may be found directly by evaporation. 


Viscosity. 


Viscosity is usually the most important property to be measured, 
and this test is carried out on a solution of fixed concentration in a 
specified solvent. The British Engineering Standards Association *° 
issue specifications for two varieties of cellulose acetate, one described 
as ‘“‘ Cellulose Acetate’? (2D. 6) and the other as “ Medium Vis- 
cosity Cellulose Acetate ’’ (D. 50). The solvent in each instance 
is made up from :— 


14 ¢.c. ethyl alcohol, 
14 c.c. benzene, 
10 c.c. methyl ethyl ketone, 
60 c.c. acetone, 
2 c.c. benzyl alcohol, 


and in this mixture 6 grammes of the sample must be dissolved. 
The viscosity at 25° is determined in an Ostwald viscometer and 
must lie between 30 and 40 for “‘ cellulose acetate,’ and between 
15 and 21 for “ medium viscosity cellulose acetate,” the standard 
substance being glycerol, which is given a viscosity in empirical 
units of 100. Barr 31 has investigated the suitability of glycerol as a 
standard in viscometry and has drawn attention to the precautions 
necessary to secure uniformity of results. 

The other routine tests are those for water content, acidity, 
ash and filming capacity. They do not call for special comment. — 


REFERENCES AND BIBLIOGRAPHY. 


1 W. L. Barnett, J. Soc. Chem. Ind., 1921, 40, 81-107. 2% J. Boeseken, 
J. C. van den Berg, and A. H. Kerstjens, Rec. trav. chim., Pays-Bas, 1916, 
35, 320-345. §% J. F. Briggs, Ann. Rep. Soc. Chem. Ind., 1918, 190. # P. 
Drinker, J. Ind. Eng. Chem., 1921,18, 835. 5 E. C. Worden, J. Soc. Chem. Ind., 
1919, 38, 3707-3747. & Soc. Chim. des Usines du Rhone, E.P. 25,893/1911. 
* Ibid., F.P. 473,399/1914 (Int. Conv. 1913). 8 Ibid., E.P. 7,773/1915 (Int. 
Conv. 1914). % Ibid., E.P. 10,822/1915 (Int. Conv. 1914). 1 Jbid., E.P. 
8,046/1915 (Int. Cony. 1914). 11 Ibid., E.P. 146,092/1920 (Int. Conv. 1919). 
12 F. Bayer & Co., E.P. 21,628/1901. 1% E. C. Worden, ‘‘ Technology of 
Cellulose Esters,” Vol. VIII., 2567-2575 (see also M. Deschiens, Chim. et 
Ind., 1920, 8, 591-607). 14 F. Bayer & Co., E.P. 24,067/1906. 45 H. Ost, 


Acetylation of Cellulose 57 


Z. angew. Chem., 1919, 32, 66-70, 76-79, 82-89. 1° C. F. Cross, J. Soc. 
Dyers and Col., 1920, 36, 19. 17 F. Bayer & Co., E.P. 14,271/1910. 18 H. 
Dreyfus, E.P. 20,977/1911. %#® H. Dreyfus, F.P. 478,023/1914. % L. 
Lederer, E.P. 3,103/1907. #1! H. 8S. Mork, U.S.P. 854,374/1907. 2? British 
Engineering Standards Association, Specifications 2D. 6 and D. 50.* 23 E. 
Knoevenagel and O. Eberstadt, Koll. Chem. Bethefte, 1921, 13, 194-212. 
24H. J. H. Fenton and A. J. Berry, Proc. Camb. Phil. Soc., 1920, 20, 16-22. 
25 See also E. Knoevenagel and K. Koenig, Cellulosechemie, 1922, 3, 113-122. 
26 Q. Torii, J. Chem. Ind. Japan, 1922, 25, 118-131. 2? G. Barr and L. L. 
Bircumshaw, Advis. Comm. for Aeronautics, Rep. and Mem. No. 303, June 
1916—-Nov. 1917. 78 Barthélemy, Monit. Scient., 1913, 78, 549. 9 M. Entat 
and E. Vulquin, Ann. Chim. Analyt., 1922, 4, 131-135. %° British Engineer- 
ing Standards Association, Specifications 2D. 6 and D. 50.* 31 G. Barr, 
Aer. Rev. Committee, Rep. and Mem. No. 755, Sept. 1920. 


Additional References. 


L. Clément and C. Riviére, ‘* La Cellulose ’’ (1920), 87-95. 

* Specifications published by the British Engineering Standards Associa- 
tion may be obtained from the Secretary of the Association, 28 Victoria Street, 
Westminster, 8.W. 1, price 2d. post free. 


CHAPTER V 
CELLULOSE ESTER SOLUTIONS: SOME PROPERTIES. PART I 


Phenomena accompanying Dissolution—Viscosity—Movement of Liquids 
in Capillary Tubes—Falling Sphere Viscometer—Dispersion—Solu- 
bility Limit—Solvent Power—Temperature Efféct—Constitution of the 
Esters—Disintegration of Structure during Esterification—Fractional _ 
Precipitation of Solutions—Dimensions of Cellulose Hydrolysed by 
Acids—Contrast with Alkaline Treatment—Chemical Constitution of 
Solvents of Cellulose Esters—Chiefly Carbonyl Derivatives—Cellulose 
Nitrate in Single Solvents—Baker’s Researches on Concentration and 
Viscosity—Later Researches—Mixed Solvents—Acetone and Water— 
Ether and Alcohol—Cellulose Acetate in Binary Mixtures containing 
Acetone—Mardles Researches —Association and De-association of the 
Solvents—Plastic Flow (Bingham)—The Tyndall Effect. 


CoLLOIDAL NATURE OF CELLULOSE EsTER SOLUTIONS 
Phenomena accompanying Dissolution. 


Ir a small quantity of dry cellulose nitrate or acetate is covered 
with a suitable solvent, e.g., acetone, the ester rapidly turns to a 
transparent jelly, which if left undisturbed will remain at the 
bottom of the vessel almost indefinitely. If the mass is stirred or 
shaken, the jelly is gradually distributed in the solution until the 
latter becomes homogeneous to the naked eye, except for small 
particles of insoluble impurities, such as incompletely esterified 
cellulose and dust, which are seldom entirely absent from the technical 
esters. If the solution is centrifuged at an early stage of the mixing, 
a considerable quantity of undispersed jelly may be thrown down 
to the bottom of the tube, but as the mixing becomes more com- 
plete the amount becomes less, and a well-mixed solution of ester 
of good solubility when whirled in a centrifuge will deposit little 
except the insoluble impurities already mentioned. The features 
accompanying the dispersion of nitrocellulose in ether—alcohol 
and acetone in particular are described by Masson and McCall,! 
and a very thorough investigation of the preliminary swelling of 
cellulose acetate in mixtures of solvent and non-solvent liquids was 
made by Knoevenagel and his pupils (v. infra). 

The most noticeable property of the solutions is their increased 
viscosity when compared with the original solvents. This is roughly 
demonstrated by tipping or inverting the vessels containing them. 
The name of “colloid”’ was originally given by Thomas Graham 
to those substances which, like glue, form viscous solutions, in which 
the dissolved substance possesses a very low rate of diffusion com- 
pared with crystalline bodies. The cellulose. esters form viscous 
colloidal solutions, known as “sols” (also after Graham), while 
under some conditions they may deposit some or all of their colloid 


content as “gels.” The nomenclature of colloid chemistry is not 
58 


Cellulose Ester Solutions: Some Properties 59 


yet quite firmly established, and colloid terms will not be used in 
this chapter more than is necessary. 


Viscosity. 

If we imagine two plane surfaces in a liquid, each of unit area 
(one square cm.), and one centimetre apart, forming as it were two 
opposite surfaces of a cube of the liquid, it is evident that to keep 
these surfaces moving relatively to each other, each remaining in its 
own plane, will require more force in some liquids than in others. 
The inherent property in a liquid which opposes this motion is 
called its viscosity, and is defined as the tangential force on a unit 
area of either of two horizontal 
planes at a unit distance apart 
required to move one plane 
with unit velocity in reference 
to the other plane, the space (a) 
between being filled with the 
viscous substance (Bingham,? 
from Clerk-Maxwell). If the 
force applied is F’, the velocity 
of one plane relative to the (b) 
other is v, and their distance 
apart s, the coefficient of vis- 

: F's 
cosity 7 = pe 

The viscosity of a liquid is 
sometimes called its internal 
friction, and the analogy with 
solid friction will be apparent, 
although it must not be taken 
too far. The viscosity of cellu- 
lose ester solutions increases at a very rapid rate with the con- 
centration, so that in plotting the viscosity—concentration curve 
it is usual to plot the logarithm of the viscosity instead of the 
actual number. This has led to the deduction of some interest- 
ing empirical equations connecting viscosity and concentration 
(v. infra). 

The simplest mechanical illustration of layers of liquid sliding 
over one another is given by deforming a pack of cards by a shearing 
stress from the shape illustrated in Fig. 1 (a) to the shape illustrated 
in Fig. 1 (b). The experimental measurement of viscosity, how- 
ever, by a method reproducing the conditions laid down in the 
definition, would be extremely difficult, and the classic method is by 


(c) 
Fia. 1. 


60 Cellulose Ester Varnishes 


the movement of the liquid in a capillary tube. It is not easy at 
first to see how this movement realises the theoretical idea of layers 
of liquid sliding over one another, but if we take as an illustration, 
instead of a pack of cards, a rolled-up tape measure, and deform it by 
pushing the centre out (cf. Fig. 1 (c)), we see how the movement of 
liquid in a capillary tube may be regarded as being made up of a 
series of concentric layers moving relatively to each other, the velocity 
being zero at the circumference (where the liquid touches the tube) 
and greatest at the centre. 

The equation connecting the radius of the tube, the head of 
pressure on the liquid, the rate of flow and the viscosity of the liquid 


2 
i897 == sai in which 
= pressure on the liquid. 
g = acceleration due to gravity. 
R = radius of capillary. 
1 = length of tube. 


I = velocity of flow (c.c. per sec.). 


It was derived experimentally in another form by Poiseuille in 1842. 
Poiseuille’s original publications are not readily available, but his 
results have been republished by Bingham, and should be con- 
sulted by all those interested in the subject. The accuracy of the 
agreement with the derived equation is on the whole remarkable. 

The measurement of rate of flow in a capillary tube is still per- 
haps the most widely used method of determining viscosity. For 
dealing with solutions of very high viscosity, however, there are 
more convenient methods. One depends on measurement of the 
rate of fall of a small steel ball through the solution, and makes use 
of the original mathematical investigation of Stokes. It was 
developed for cellulose ester solutions by Sheppard * and by Gibson 
and Jacobs.* Others measure the actual friction when one metal 
cylinder is rotated inside another concentric with it, the space 
between being filled with the liquid under investigation. Measure- 
ments of the viscosity of cellulose esters for technical purposes are 
usually made either with the Ostwald (capillary tube) (B.E.S.A.) 
or the falling sphere instruments. 


Dispersion. 

Strictly speaking, the term solution should be applied only to 
those mixtures in which the components are subdivided down to 
molecular limits. Viscous colloids are believed to subdivide into 
particles much larger than molecules, and they are therefore said to 


Cellulose Ester Solutions: Some Properties 61 


disperse rather than to dissolve, and to form dispersions rather than 
solutions. The term “ colloid solution,’? however, it used even in 
purely scientific literature when the context provides against mis- 
conception. It is becoming customary to distinguish between 
“solution ’’ and “ dissolution,” the latter referring to the process of 
dissolving, and the former to the product. ‘‘ Dispersion” at 
present carries both meanings, but “ spersion ’’ may arrive in due 
course. 
Solubility Limit. 
~ When a crystalline substance, e.g., sodium chloride, is dissolved 
in water, it is common knowledge that if the quantity of salt is 
increased, a point is reached at which the liquid, at any fixed tempera- 
ture, will dissolve no more of the solid. An alteration in temperature 
will usually cause an alteration in the quantity that can be dis- 
solved, but the limiting concentration is just as sharp. The cellulose 
esters behave differently. If we continue to add cellulose nitrate 
to acetone, it will disperse with more and more difficulty owing to 
the mechanical difficulty of stirring the viscous mass, but we do 
not reach a point at which the acetone is saturated with cellulose 
nitrate and cannot dissolve any more. The limit appears to be set 
only by the mechanical strength of our apparatus. Hence we cannot 
compare the technical advantages of different solvents of a cellulose 
ester by measuring the concentration of the ester at saturation 
point, as we should do if we were dealing with a crystalline sub- 
stance; other methods have to be used. The most obvious one is 
suggested by the earlier part of this paragraph. If the dispersion 
of the ester in the solvent is limited only by the mechanical strength 
of the apparatus, that solvent is the best (in regard to solvent power 
only) which at equal concentration gives the least viscous, and 
therefore most easily workable, solution. Hence one way of 
comparing solvent power is to measure the viscosity of solutions 
of the same cellulose ester at the same concentration. 
Another method arises indirectly from economic considerations. 
It is often possible, under conditions which will be discussed later, 
to dilute solutions of a cellulose ester in a solvent with a cheap 
non-solvent. For example, a solution of cellulose nitrate in amyl 
acetate may be diluted with petroleum spirit to a considerable 
extent without causing precipitation of the dissolved ester. Hence 
one may compare two solvents, such as, for example, amyl acetate 
and ethyl acetate, by preparing from each a solution of cellulose 
nitrate of identical concentration, and determining what proportion 


62 Cellulose Ester Varnishes 


of petroleum spirit may be added to each, at any chosen tempera- 
ture, before precipitation begins. 


Correlation of Solvent Power and Viscosity of Cellulose Ester 
Solutions. 


This experiment determines the ‘‘ solvent power number,” which 
has been defined by Mardles ® as the volume in cubic centimetres 
of a miscible non-solvent required to start precipitation of a cellulose 
ester from 1 gramme of a 5% solution at 20°. — 


NS 


Viscosity —> 
aS © 
- ——— Solvent Power Number. 


A) 





80° 70 60 580° 40° 30 202-7 O 
Alcohol per cent. ~ 


Fie. 2 (Mardles).—The curves connecting viscosity and solvent power number 
with the composition of the solvent are here plotted for a solution of nitrocellulose 
in ether-alcohol. Note that each curve displays a decided minimum at approxi- 
mately 50—60% of alcohol in the solvent, and that viscosity is measured upwards 
and solvent power number downwards. Thus minimum viscosity corresponds 
approximately with maximum solvent power, 7.e., maximum resistance to pre- 
cipitation by a diluent. 


It has been shown by Mardles © that there is a near relation 
between the viscosity of a solution of cellulose ester and the solvent 
power, and if these two properties are measured for solutions of 
cellulose esters in mixtures of two liquids in varying proportions, 
the curves expressing the results are generally similar in shape, 
z.e., these two methods of estimating the solvent value of a liquid 


Cellulose Ester Solutions: Some Properties 68 


usually show moderately good agreement (see Fig. 2). Mardles 
points out that on theoretical grounds the best solvent mixture 
should be the least viscous (1.e., the most fluid), since high fluidity 
indicates a considerable degree of dispersion and therefore a close 
approach to true solution. The industrial value of these measure- 
ments is evident from the fact that the solvents of highest solvent 
power are those which dissolve the esters most rapidly, and which 
give solutions of required viscosity with the smallest volume of 
solvent, while the solutions are the clearest and have the least 
tendency to coagulate. 


Effect of Temperature. 


Although there is no definite limit to the amount of cellulose 
ester which can be dissolved in a given solvent, there exists for many 
substances a temperature above which they are solvents and below 
which they are not. This critical temperature has been termed the 
transition temperature.’ It is not quite sharp, probably owing to 
the heterogeneity of commercial cellulose esters, and it varies 
slightly with the concentration, so that the concentration of the 
solution should be stated when quoting it. The usual concentration 
employed for the determination is 59%. Increase of temperature 
always lowers the viscosity of solutions. 


Chemical Constitution of the Cellulose Esters. 


Before considering the results obtained in the investigation of 
single and mixed solvents, a few words should be said about the 
constitution of the esters themselves. 

It will be recalled that cellulose has been proved to be built up 
of glucose anhydride residues, having the formula :— 


The groups B and C are themselves built up of similar glucose 
units. This formula contains three hydroxyl groups in every C, unit, 
and we may write it as [C,H,O,(OH)3)n. These hydroxyl groups may 
theoretically be esterified in three stages, yielding the compounds 

C,H,0,(0H),(OB) 
C,H,0,(0H) (OR), 
C.H,0,(OR), 


64 Cellulose Ester Varnishes 


R being CH,:CO in the acetic esters and NO, in the nitric esters. 
In the following table are shown the percentages of hydroxyl group, 
of acetyl group as acetic acid and of nitryl group as nitrogen in the 
theoretical cellulose group as the hydroxyl groups are successively 
esterified :— 








% OH % acetic On ASEL YN 
in acid in in in 
acetate. acetate. nitrate. nitrate. 
Celloee |; crises cag ccteabendates [31-5] nil [31-5] nil 
- mono-ester ......... 16-7 29-4 16-4 6:8 
oe EEE ORES ae a ates 6:9 48-8 6°7 11-1 
ve, ETS OMEN! eB ak nil 62-5 nil 14-1 


It must be emphasised that these figures refer only to a theoretical 
esterification in which there has been no simultaneous hydrolysis of 
anhydride oxygen (—O—) atoms in the cellulose. 

Thus the theoretical cellulose di-nitrate contains 11:1% of 
nitrogen and 6:7% of hydroxyl. The corresponding di-acetate con- 
tains acetyl groups equivalent to 488% of acetic acid and 6-9% of 
hydroxyl, while the tri-acetate contains 62-5°% of acetic acid and no 
hydroxyl. It is evident therefore that, except in the instance of 
completely esterified cellulose, there is sufficient free hydroxyl in 
the ester to lead us to expect some evidence of its presence in the 
properties of the material. We know, moreover, that esterification 
of cellulose is always accompanied by some degree of hydrolysis 
which increases the percentage of hydroxyl group to more than 
the theoretical value. 

It has already been mentioned that the products of nitration and 
acetylation of cellulose do not indicate directly the esterification of 
exactly one, two or three hydroxyl groups. Stable cellulose nitrates 
may be obtained with any percentage of nitrogen from about 10% 
to nearly 14%. This has been explained on the hypothesis that the 
ester groups are not chemically combined with the cellulose, but 
adsorbed. This theory, however, has had little or no support, and 
the usual view is that all the C, units need not necessarily be esterified 
to the same extent. Some, for example, may be di-nitrates, others 
tri-nitrates, the average percentage of nitrogen for the nitrate in 
bulk corresponding to something between a di-nitrate and a tri- 
nitrate. Another consideration which has been lost sight of to 
some extent is that since hydrolysis takes place simultaneously with 
esterification, the percentage of nitrogen must thereby be altered. 


Cellulose Ester Solutions: Some Properties 65 


For instance, if an oxygen linkage between two neighbouring C, 
units is hydrolysed by addition of water, simultaneously with the 
esterification of two hydroxyl groups in each unit, the percentage 
of nitrogen in the two units considered together will not be 11-1%, 
as in the theoretical di-nitrate, but 10-4%. This is, of course, an 
extreme case, but it is evident that some degree of hydrolysis dis- 
tributed over a comparatively large conglomerate of C, units will 
cause the percentage of nitrogen in a true di-nitrate to diverge 
appreciably from 11:1%. 


Disintegration of Cellulose Structure during Esterification. 


We have already seen that the degree of polymerisation of the 
cellulose unit, or, in other words, the value of m in the formula 
(C,H,)0;). is unknown. It has been suggested that the entire 
cotton hair may be a single ‘“‘ molecule”’ of cellulose, and it must 
be admitted that the larger the structure grows, the more difficult 
it is to see what internal forces can suddenly come into play to 
stop growth. The limit is probably enforced by conditions external 
to the plant, such as temperature and especially humidity. Thus 
it has recently been shown ® that there is a large variation in the 
viscosity of nitrocellulose prepared from wood cellulose which has 
been extracted from concentric rings of the same section of the 
trunk of a poplar tree. The central zone yielded nitrocellulose 
with the lowest viscosity and the outer zone nitrocellulose with 

the highest viscosity. 
The high initial viscosity of a solution of cotton cellulose which 
has not been exposed to light or oxidation is attributed by Barr ° 
to the existence of an outer cuticle of cellulose more highly poly- 
merised than the rest of the cotton hair. 

It is natural to ask to what extent the structure is disintegrated 
during esterification. It is not likely that, even if the structure 
of cotton cellulose is coterminous with the cotton hair, it will nitrate 
as a hair, although experiments are on record in the celluloid 
industry in which transparent celluloid gradually developed cloudi- 
ness due to the appearance of fibres.1° Prolonged nitration or 
acetylation always give products of low viscosity, and there is 
little doubt that hydrolysis and esterification begin simultaneously 
with the immersion of the cotton in the acids. The product of a 
nitration probably consists of fragments of the original cellulose, 
not of uniform size, of which the smaller ones have probably under- 
gone a greater proportionate degree of hydrolysis than the larger. 
The Sua picture of the esterification process is assisted if we 


66 Cellulose Ester Varnishes 


remember that according to Balls 41 the wall of the cotton hair is 
probably a sponge-like structure containing free air spaces, and 
therefore presenting a large surface to the esterifying acids. 


Fractional Precipitation of Solutions. 


The presence of units of different dimensions in cellulose esters 
is clearly shown by the experiments of Duclaux and Wollman,” — 
who fractionally precipitated a solution of cellulose nitrate in 
acetone by means of aqueous acetone, and found that although 
the various fractions were practically identical in nitrogen per- 
centage, they differed greatly in viscosity, the earliest fraction 
(i.e., the first fraction to precipitate) having the highest viscosity, 
and succeeding fractions diminishing progressively in viscosity. 
Since it is unlikely that the method of precipitation could have 
any influence on the molecular dimensions of the dissolved particles, 
we must conclude that these various cellulose nitrates of different 
viscosity co-exist in the original solution, and that the least viscous 
are the most soluble in aqueous acetone. Cellulose acetate has 
been found in the writer’s laboratory to behave similarly. 

The acetylation of cellulose has been the subject of interesting 
research from a similar point of view by Béeseken, van den Berg, 
and Kerstjens.1® The formula for cellulose may be written 


(CH 120,4)n — (n — 1)H,0, 


in which n diminishes as the structure is hydrolysed, while the 
percentage of acetic acid rises from 62-5% for cellulose tri-acetate 
to 77% (for dextrose pentacetate, 7.e., after complete hydrolysis to 
dextrose). Bdéeseken and his colleagues first found that if acetyla- — 
tion were stopped before the cellulose had gone into solution, the 
acetate found in solution was the tri-acetate, and the undissolved 
cellulose was practically unesterified. They therefore concluded 
that when acetylation occurred the tri-acetate was formed imme- 
diately. When cellulose (C,H,)0;)n is hydrolysed by the addition 
of one molecule of water, its molecular weight becomes 
(C,H 1)0;)2 + 18, and the number of hydroxyl groups becomes 
3n + 2. Hence 3n + 2 molecules of acetic acid will be required 
for the acetylation to tri-acetate, and, since one molecule of water 
is lost for each hydroxyl acetylated, the molecular weight of the 

product will be given by the formula . 


[(CgH 4 05)n + 18] + (8” + 2)(CH;;COOH) — (3n + 2)(H,0) = 
3 [288m + 102] 


Cellulose Ester Solutions: Some Properties 67 


The percentage of acetic acid in this tri-acetate is 


100 x 60(387 + 2) _18,000n + 12,000 


288m +102 ~~ 288n + 102 





and since this constituent can be determined analytically, the value 
of m can be found; it gives the average number of condensed 
dextrose groups in the products of hydrolysis. When n is very 
large, the percentage of acetic acid in the product closely approaches 


ale 2 or 62:5%. Asn becomes smaller, the value approaches 77%. 





It was found by following the course of acetylation that the experi- 
mental number did not rapidly increase above 62:5%, showing 
that the cellulose had probably at first hydrolysed into fragments 
approximately equal in size. This was confirmed by the fact that 
only a small proportion of the product dissolved in ether—alcohol, 
which dissolves the acetates of the carbohydrates of low molecular 
weight. When once the increase in combined acetic acid begins, 
however, it becomes rapid, and indicates that the cellulose structure 
is being disintegrated quickly. 

Barnett 14 found that cellulose acetates contain free ketonic 
groups which react in the normal way with phenylhydrazine or 
p-bromophenylhydrazine, often forming compounds of definite 
melting point. By determining the percentage of nitrogen or 
bromine in these compounds he was able to deduce values for n, 
which he found to vary from 2 to 12 for the phenylhydrazones 
and from 9 to 36 for the bromophenylhydrazones. The method is 
useful for showing progressive degradation of the cellulose structure 
as the acetylation proceeds, but many of the acetates studied appear 
to have been considerably further depolymerised than those used 
in industry. 

[It is interesting to compare with these results those obtained 
by Irvine and Hirst 15 in the methylation of cellulose, in an 
alkaline medium by the method of Denham and Woodhouse. 
Fourteen methylations yielded a product containing 43-89% OCH, 
and twenty more methylations produced no further rise in the 
percentage of methoxyl groups. Irvine concluded that methylation 
caused no hydrolysis or oxidation of the cellulose molecule. With 
this conclusion may be compared the results obtained by Punter 14 
and his collaborators on a very different scale, in the technical 
treatment of cotton required to produce uniform viscosity. They 
found that hot alkaline treatment of different varieties caused 
reduction of viscosity, and therefore presumably of structure, but 


68 ~ Cellulose Ester Varnishes 


after a time all varieties appeared to be reduced to the same more 
stable structure, the viscosity of which was not altered by further 
treatment of the same kind, as long as no chemical breakdown 
took place. Something of the same nature may have happened 
to Irvine’s methylated cellulose in the early methylations, so that 
the formula which he derived may be that of a cellulose modified 
by the alkaline treatment. ] 

The evidence therefore suggests that the acetylation or nitration 
of cellulose yields a product which consists of fragments of the 
original cellulose structure, probably not esterified uniformly as 
regards the number of ester groups in each C, unit, not of uniform 
dimensions, and therefore not uniformly hydrolysed, but perhaps 
not differing very largely in this respect. 


Chemical Constitution of Solvents. 


It has been mentioned in the first chapter of this book that 
the earlier developments of cellulose nitrate solutions were much 
hampered by the lack of suitable solvents. As soon as it was 
realised that there were vast commercial possibilities in such solu- 
tions, a large number of substances were tested and many of them 
patented. De Mosenthal 1’ gives a long list and Worden 1* one 
still longer. It has been said quite recently by a critic that the 
method of research seems to be one of trial and error. There are, 
however, some regularities which give indications of the constitution 
and properties which confer solvent power for cellulose esters. 
Cellulose nitrate is dissolved by the lower ketones, e.g., acetone, 
methyl ethyl ketone, diethyl ketone; by the methyl, ethyl, propyl, 
butyl and amy] esters of formic, acetic, propionic and butyric acids ; 
and by various derivatives of acetanilide. Cellulose acetate is 
dissolved by acetone, methyl acetate, methyl and ethyl formate, 
formic and acetic acids. We find, however, an extremely inter- 
esting class of solvents prepared by mixing liquids which, by them- 
selves, have little or no solvent power. For example, for cellulose 
acetate: chloroform and alcohol, tetrachloroethane and alcohol, 
benzene and alcohol (hot). For cellulose nitrate: ethyl ether and 
alcohol, benzene and alcohol, toluene and alcohol. Since, for 
technical purposes, cellulose esters are almost always employed — 
in mixed and not single solvents, the investigation of this pheno- 
menon is of great technical interest, and during the war, owing to 
the shortage of solvents and the imperative necessity of reducing 
the demands on shipping, the research became a national necessity. 
If we examine the lists of cellulose ester solvents, we notice that 


Cellulose Ester Solutions: Some Properties 69 


single solvents always contain some atomic grouping of marked 
reactivity. Consider, for example, the list of cellulose nitrate 
solvents given above. These substances are all either ketones, 
esters, or anilides, the distinctive groupings of which are 

R,—CO—R, 

R,—CO—OR,, 

Ar—NH—CO—R, 

respectively, the carbonyl group —CO— being common to all of 
them. 


Cellulose Nitrate in Single Solvents. 


The first systematic examination of single solvents for cellulose 
nitrate was made by Baker,!® who studied the relation between 
viscosity and concentration, using three different typical samples 
of the material. The solvents used were :— 

Acetone, CH,°CO-CH; ; 

Ethyl formate, H-COO-C,H,; ; 
Methyl acetate, CH,-COO-CH;, ; 
Ethyl acetate, CH,-COO-C,H,; ; 
Propyl acetate, CH,°COO-C,H, ; 
Amy] acetate, CH,-COOC,H,, ; 
Ethyl butyrate, C,H,;->COOC,H,; ; 


vA Nee Cols (the phenyl analogue is 
Aceto ethyl-o-toluidide,|_ [~ ~CO°CH, mannol) 
neg 


CH, 
/\wc eh 
Ethyl-o-tolylethyl carbamate, | CO-OC,H; 
| Se 


db \w Cells 
Phenyl ethyl urethane,| [| ~CO°OC,H; 
I 


COOC,H,; 

Ethyl phthalate, | =| ; 
“\C00-0,H, 

Baker found that if concentration were plotted against viscosity 


at constant temperature, the curve for each solvent could be 
represented by the empirical formula 


1 = N (1 + ac) 
n = viscosity of solution at concentration ¢ 


No = Viscosity of solvent 
a, k = constants, different for each solvent. 


where 


70 Cellulose Ester Varnishes 


A convenient expression for comparing different solvents is 
given by the differential 


d log of A constant 
dc 


The value of this constant differs for the same nitrocellulose in 
different solvents and hence the effect of cellulose nitrate on the 


viscosity of its solutions varies with the nature of the solvent. 
Probably also the condition of the nitrate is not the same in different 





5 


Log (7x 10°) 
® 





O OS aor 15 2:0 
Concentration per cent, 


Fia. 3 (Baker).—Concentration of nitrocellulose plotted against the logarithm 
of the viscosity, for a solution in equal volumes of benzyl alcohol and ethyl ether. 


If n =n (1 + ac) 
eect Cael ee 


and ak = 2-303 tan a, where a is the angle between the tangent and the concentration 
axis. The value of ak is a characteristic function of each curve (Baker, loc. cit.). 


liquids. Hence the act of solution produces some change in the 
cellulose nitrate other than a merely physical change of state. 
Baker concluded that the cellulose nitrate becomes associated with 
the solvent, so that the true solute is not cellulose nitrate, but a 
complex of cellulose nitrate and the solvent. He accepts a sug- 
gestion previously put forward by Schwarz *° from a study of the 
viscosity of cellulose nitrate in camphor and alcohol, that the 
fluidity (7.e., the inverse of viscosity) of a solution of cellulose 


\ 


Cellulose Ester Solutions: Some Properties ‘71 


nitrate in a solvent is a criterion of the gelatinising or solvent power 
of the liquid. 

Results similar to Baker’s were obtained by C. Visser 24 by the 
examination of cellulose acetate from three different sources in the 
simple solvents acetone, methyl acetate and ethyl formate. Her 
equation, which is also derived empirically from an examination 
of the curves, is of the same form as Baker’s, namely, 


Le no( 1 H a) 


where P and Q are constants, and she adopts the suggestion of 
Baker and of Hatschek ** that the aggregates in the solution 
consist of the dissolved ester combined in some way with part 
of the solvent, so that the disperse phase is a complex depending 
on the grade of cellulose acetate and the nature of the solvent. 
This paper gives a brief discussion of the formule suggested PY 


Hinstein (Ann. Physik, 1906, (iv), 19, 289). 

Hatschek (Z. Chem. Ind. Koll., 1910, 7, 301-304; ibid., 1911, 
8, 34-39). 

Bancelin (Compt. rend., 1911, 152, 1382). 

Kendall (Medd. K. Vet. Nobelinst., 1913, 2, No. 25, 1-16). 

Arrhenius (Z. Phys. Chem., 1887, 1, 288). 

Baker (T'rans. Chem. Soc., 1913, 108, 1653-1675, v. supra). 

Smoluchowski (Koll. Z., 1916, 18, 190). 


connecting viscosity and concentration. 

Duclaux and Wollman ™ arrived at a still simpler equation | 
connecting concentration and viscosity, namely :— 

1) = 19 - 10", 
and state that & is almost independent of the solvent for the same 
nitrocellulose, but differs greatly with different nitrocelluloses so 
that it forms a kind of index of the viscosity of cellulose nitrate. 
It will be noted that if the viscosity of a sample of cellulose nitrate 
is 7’ in solvent A, and 7’ at the same concentration in solvent B, 
then 
Meet Le Ge as Oe me ere 


meta, A i ees op a ee aay en 


If & is independent of the solvent, we obtain by dividing equation 
(1) by equation (2) 


and 


72 Cellulose Ester Varnishes 


1.€., the viscosities of solutions of equal concentrations are in the 
same ratio as the viscosities of the solvents. This relation would 
certainly not hold over a wide range of different solvents and 
different nitrocelluloses, but the results given by Duclaux and 
Wollman are of considerable interest, and it would be a valuable 
research to determine the limits of the applicability of the equation. 
If it was derived only from the examination of the products of the 
fractional precipitation of solutions of cellulose nitrate in acetone, 
it suggests that these fractions may show regularities (perhaps 
due to more restricted range of dimensions) which are not shown 
by samples derived from entirely different processes of nitration. 


CELLULOSE Esters In MIxED SOLVENTS. 
Cellulose Nitrate in Acetone and Water. 


This system was examined by Masson and McCall,1 who found 
that the viscosities of solutions in anhydrous acetone are com- 
paratively high, but are lowered by the presence of small quantities _ 
of water. A minimum value exists which corresponds to a per- 
centage of water which is slightly different for different samples 
of nitrocellulose, and also varies somewhat with the concentration 
for the same samples of nitrocellulose (see Fig. 4). For example, 
using cellulose nitrate containing 12-39% of nitrogen, the minimum 
viscosity for a 5% solution occurs at 8—9% of water, and for a 
10% solution at 9—10% of water. Using cellulose nitrate con- 
taining 13:0% of nitrogen (gun-cotton), the minimum viscosity 
for a 5% solution occurs at 6—7% of water and for a 10% solution 
at 7°%% of water. If the amount of water is increased much beyond 
these figures, a point is reached at which gelatinisation still occurs, 
but the cellulose nitrate will not disperse. 

As a matter of interest it may be mentioned that at still higher 
concentrations of water a mixture is reached which when shaken 
with fibrous cellulose nitrate will powder it without dissolving it 
(although a small loss of weight occurs). This process is an 
approach to the fractionation process of Duclaux and Wollman ™@ 
from the opposite direction. 


Ether and Alcohol. 


When it is found that certain varieties of cellulose nitrate 
dissolve in a mixture of ether and alcohol, neither of which is a 
solvent alone at ordinary temperatures, nor contains an atomic 
grouping which would lead us by analogy to expect that it would 


Cellulose Ester Solutions: Some Properties 78 


be a solvent, it is natural to speculate whether, when the two solvents 
are mixed, any complex is formed which rearranges their affinities. 
Baker 7 investigated pure ether—alcohol mixtures from this point 
of view, using the sensitive property of viscosity as a test for 
complex formation. He concluded that mixtures of alcohol and 
ether contain 


(a) non-associated ether, 
(>) ether—alcohol complex, 
(c) non-associated alcohol, 
(d) associated alcohol, 





ye 


‘Scosi 


e 
4 


V, 


O 2 4 6 8 10 
Per cent Water in Acetone. 


Fic, 4 (Masson and McCall).—These curves show the viscosity of three solutions 
of nitrocellulose in mixtures of acetone and water, plotted against the composition 
of the solvent. Note that, in each example, the lowest viscosity is shown, not in 
anhydrous acetone, but in acetone containing an appreciable quantity of water. 
The concentration of nitrocellulose is constant along each curve. 

‘ar 1.—10 grammes of nitrocellulose (12:39% nitrogen) in 100 grammes of 
solvent. 

ere. 2.—8:5 grammes of nitrocellulose (12-39% nitrogen) in 100 grammes of 
solvent. 5 

rings 3.—10 grammes of nitrocellulose (13:0% nitrogen) in 100 grammes of 
solvent. 


and that the solvent power of the mixture is due to the complex. 
Baker rejected the view that the solvent power was due to 
unassociated alcohol molecules, and that the effect of the ether 
was merely to increase the number of those molecules, on the following 
grounds :— : 


re! Cellulose Ester Varnishes 


(a) It is not true that the higher the degree of association 
in a homologous series of alcohols, the less is the solvent power. 
The opposite is the case. 

(6) If increase in the proportion of unassociated alcohol 
increases the solvent power, any indifferent liquid added to 
alcohol should increase the solvent power, but this is not so. 


He also attributed the increased solvent power of ether—alcohol 
at low temperatures *4 to the increase in the proportion of complex 
at these temperatures. 

Gibson and McCall 2° investigated the effect of variations in the 
proportions of ether—alcohol on the viscosity of solutions of cellulose 
nitrate, and found that the proportions required to produce the 
best solvent (i.e., as shown by the lowest viscosity) depended on 
the nitrogen content of the nitrocellulose, and that the higher the 
nitrogen content the greater the proportion of ether required in 
the solvent (Fig. 2). 

From the discussion given in the earlier part of this chapter 
it will be evident that variation in nitrogen content may be, and 
probably always is, accompanied by independent variation in— 


(a) the degree of hydrolysis of the cellulose, and therefore 
in the percentage of hydroxyl groups, 

(b) the average dimensions, and the range of dimensions, 
of the nitrated cellulose particles, 

(c) the percentage of the minor functions of the cellulose, 
of which only the sulphuric acid ester group need be mentioned 
here. 


Therefore the proportion of ether required to produce the best 
solvent may be influenced, not only by variation in the nitrogen 
content, but also by variation in any of these accompanying factors. 
If, for instance, one sample of cellulose nitrate contains a higher 
percentage of hydroxyl groups than another, and if we assume 
that these attract alcohol molecules in preference to ether molecules 
from the ether—alcohol mixture, we should have a reason why 
the sample with higher hydroxyl content should require a solvent 
with a higher proportion of alcohol, and vice versa. This conclusion 
is not inconsistent with Baker’s theory that the active solvent is an 
ether—alcohol complex. Highfield ** criticises this view on several 
grounds. In the first place, working with ether—alcohol solutions 
containing from 10 to 60% of ether, the variation in viscosity is 
much less when only 2% of water is present than when 7% is 
present, so that variation in the amount of ether—alcohol complex 


# 


Cellulose Ester Solutions: Some Properties 75 


is not a predominating factor. Secondly, Gibson and McCall found 
that a 4% solution of cellulose nitrate containing 11-8° of nitrogen 
required a solvent of 50% alcohol to yield a solution of minimum 
viscosity, while if the nitrogen content were 12:5%, the required 
proportions were 70% ether and 30% alcohol. This implies that an 
increase of 0-7% in the nitrogen content involves 20% of the alcohol 
in these 4°% solutions, so that in making more concentrated solutions 
so much alcohol would be used up in saturating the hydroxyl groups 
of the cellulose nitrate that little would be available for producing 
the ether-alcohol complex. Therefore it should be difficult to 
make these more concentrated solutions, which is not the case. 
Thirdly, if part of the alcohol is required to saturate hydroxyl 
groups, the composition of the best solvent mixture should vary 
with the amount of cellulose nitrate to be dissolved, but, in fact, 
for the same sample, the proportions of ether—alcohol required 
to give the lowest viscosity are independent of the concentration. 
Lastly, if solvent power depends on an ether—alcohol complex, one 
would expect dry ether—alcohol to yield solutions of lower viscosity 
than those containing water, whereas, in fact, dry ether—alcohol 
mixed with cellulose nitrate produces stiff jellies which become 
much more fluid on the addition of a little water. 

These objections must be taken into account in constructing 
any picture of the dissolution of cellulose nitrate by ether—alcohol. 
It should be pointed out, however, that the suggestion of the 
selective adsorption of alcohol from ether—alcohol by the hydroxyl 
groups of dissolved cellulose nitrate forms no part of Baker’s theory 
of a solvent ether—-alcohol complex. The influence of the affinity 
of cellulose nitrate for alcohol molecules, however, cannot be dis- 
regarded. Everyone with practical experience of the dehydration 
of cellulose nitrate with alcohol knows what differences there are 
between different types of material in the tenacity with which 
alcohol is retained. Obviously, however, this affinity is only 
one of several co-existing in equilibrium in an ether—alcohol solution 
containing water. If one sample of nitrocellulose attracts alcohol 
molecules more than another, the equilibrium will be displaced 
in such a direction that a higher proportion of alcohol will be 
required in the best solvent mixture. This view is qualitatively 
in agreement with the theory of the existence of an ether—alcohol 
complex, but is quite independent of that theory, and we are far 
from possessing sufficient data to express the changes in equilibrium 
quantitatively. 

Masson 2? points out that the influence of temperature must 


76 Cellulose Ester Varnishes 


be considered in any general theory. Alcohol or, still better, 
aqueous alcohol at — 80° will gelatinise cellulose nitrate, whereas 
ether will not. In an ether—alcohol solution, we do not know how 
the solvent is distributed between the dispersed cellulose nitrate 
and the dispersing medium, and we cannot deduce the composition 
of the phases from the initial composition of the solvent. There 
is some mutual action between ether and alcohol which favours 
absorption of these liquids by cellulose nitrate, but at present its 
nature is undetermined. 

A different aspect of ether—alcohol solutions is opened up by 


3 
a 


Relative Viscosity. 
a 


“=~ 
i) 
H 





20 30 40 
Molecular Percentage of Water. 

Fic 5, (Barr and Bircumshaw).—This curve shows the viscosity of a sample of 
Dreyfus cellulose acetate in solvents consisting of various mixtures of acetone and 
water, the concentration of cellulose acetate being maintained constant at 5%. 
The viscosities are measured in empirical units and are here only of relative value. 
Note that the minimum viscosity is shown in a mixture containing about 20 mole- 
Soe water %. Compare behaviour of cellulose nitrate in acetone and water, 

ig. 4. 

Kugelmass,?° who exposed a sample of cellulose nitrate containing 
11:9% to the action of alcohol and ether separately at low tem- 
peratures. In each case a sol was formed above the layer of 
undissolved nitrocellulose. The opalescent ether sol contained 
nitrocellulose with 13-75% of nitrogen. It was centrifuged until 
no more solid was deposited, and the cellulose nitrate obtained 
from it contained 11-20% of nitrogen. The experiment with 
alcohol carried out in the same way yielded a product with 14-02% 
of nitrogen. The theoretical di-nitrate requires 11-13%, and the 
tri-nitrate 14-17%, so that apparently at low temperatures, ether 
disperses the one and alcohol the other. 


a see 


/ 


Cellulose Ester Solutions: Some Properties ‘ii 


Cellulose Acetate in Mixed Solvents. 


The solubility relations of cellulose acetate have probably 
been studied more completely than those of cellulose nitrate. This 
is due to two causes: (1) the shortage of appropriate solvents during 
the war, (2) the more restricted range of solubility shown by cellulose 
acetate as compared with cellulose nitrate. 


1500 


: 


1000 


750 


Relative Viscosit Y: 





Oo 


& ra) 15 ZO 25 
Molecular Percentage of Benzene. 


Fic, 6 (Barr and Bircumshaw).—This curve is directly comparable with that 
shown in Fig. 5, and shows the viscosity of the same sample of cellulose acetate in 
solvents consisting of various mixtures of acetone and benzene, the concentration 
of cellulose acetate being maintained constant at 59%. Note that there is no minimum 
viscosity. Benzene, unlike water, acts throughout merely as a diluent and increases 
the viscosity of the solution. 


Barr and Bircumshaw *° examined the changes in viscosity of 
solutions of cellulose acetate in acetone to which varying quantities 
of water (Fig. 5), alcohol and benzene (Fig. 6) had been added. 
The concentration of acetate in the solutions was 5%. The addition 
of alcohol causes a considerable fall in the viscosity, reaching a 
minimum at about 6% of alcohol and then remaining moderately 
steady up to a concentration of 40%. Aqueous acetone yielded 
solutions which had a well-marked minimum viscosity when the 


78 Cellulose Ester Varnishes 


acetone contained 19% (mol.) of water. The addition of benzene 
to acetate solutions, on the other hand, caused a steady increase 
in the viscosity of solutions, the curve showing no minimum. 

In a number of instances, observers have found minima in the 
viscosity—concentration curves which approximated closely to a 
simple molecular combination of the constituents of the mixture, 
€.9.; 

1 mol. of tetrachloroethane + 1 mol. of alcohol 
2 mols. of epichlorohydrin -+ 1 mol. of alcohol 


1 mol. of » + 2 mols. of acetic acid ‘ 
1 ,, of ethyl formate + 1 mol. of acetone 
1 ,, of mesityl oxide +1 ,, of alcohol. 


Mardles * has therefore examined several examples of mixed solvents 
in which different degrees of affinity would be expected between 
the constituents. Aniline and acetic acid, for example, form a 
solvent mixture for cellulose acetate, but also, as a basic and acid 
compound respectively, react with each other forming compounds 
which can be isolated. A viscosity—concentration curve for the 
solvents alone shows a marked peak, 7.e., a maximum viscosity 
at a position close to, but not coinciding exactly with, the point 
corresponding to a compound of 2 molecules of acetic acid with 
1 molecule of aniline. If the same curve is plotted for similar 
mixtures containing a fixed amount of cellulose acetate in solution, 
the viscosity maximum is much exaggerated, and is shifted a little 
nearer to the position of molecular ratios. The curve connecting 
solvent power number with concentration shows a similar maximum. 
In this solvent mixture there is no doubt that a complex is formed 
between the two constituents of the mixture, and that the solvent 
power is diminished by the formation of this complex. 

The corresponding curves for a mixture of benzyl alcohol and 
cyclohexanone show a sag and not a peak, and again the char- 
acteristic of the curve is much exaggerated when the mixtures 
contain cellulose acetate, the sag becoming a V. 

The mixture of acetic acid and water shows a well-marked 
maximum solvent power for cellulose acetate at a concentration 
of about 36% of water for a 5% solution of cellulose acetate, and 
the mixture of acetic acid and methyl alcohol a similar maximum 
at a concentration of about 24% of methyl alcohol. Nevertheless, 
both of these mixtures show evidence of the formation of a com- 
plex between the two ingredients. Hence we have the anomaly 
that a non-solvent, water, added to a solvent, acetic acid, (a) forms 
a complex with the acid, but (b) increases its solvent power. At 


Cellulose Ester Solutions: Some Properties 79 


first sight this appears to bring us back to Baker’s theory of 
the solvent power being due to the complex, but in view of all the 
evidence, Mardles concludes that there are instances in which the 
addition of one liquid to another may result both in complex 
formation and in molecular simplification (de-association). The 
increased solvent power is due to the solvent action of the simple 
molecules produced by de-association of associated molecules. 
Complex-formation decreases the solvent power, but when complex- 
formation and de-association occur simultaneously, the favourable 
influence of the later phenomenon may entirely mask the unfavour- 
able influence of the former. 

A few words may be said about other physical properties the 
study of which has not yet given results comparable with those 
obtained by studies of viscosity and solvent power in so far as 
the commercial utilisation of cellulose ester solutions is concerned, 
but which will have to be considered in any general theory of their 
constitution. 


Viscosity and Plastic Flow. 


The experimental and mathematical investigations of Bingham 
in the United States have stimulated research on the properties 
actually concerned in viscosity measurements of colloids. It 
follows from the equation given on p. 60 that if the same liquid is 
used in the same capillary tube, 

ip == bf, 
i.e., the curve connecting pressure with outflow is a straight line 
passing through the point of origin. Bingham and his collaborators 
have investigated a large number of colloidal solutions in visco- 
meters permitting variation of pressure, and have found that the 
p/I curve at low pressures is not linear. At higher pressures it 
becomes linear, and the straight line if produced cuts the pressure 
axis at a small positive value, 7.e., the linear part of the curve is 
represented by a modified equation (P — p) = k I, where P is the 
total pressure applied, and p, the intercept on the pressure axis, is a 
fraction of the total pressure, which is used up, according to Bing- 
ham, in overcoming an internal friction or resistance to viscous flow. 
This behaviour distinguishes such solutions from pure liquids. 
Solutions of cellulose nitrate in acetone were studied by Bingham 
and Hyden,®° and were found to exhibit the property just described. 
At temperatures from 5° to 40° the isotherms connecting pressure 
and outflow are straight lines which by extrapolation cut the pressure 
axis at points representing small positive values. These small. 
pressures, which must be applied before viscous flow begins, are 


80 Cellulose Ester Varnishes 


termed the yield values. The yield value is not the same for 
solutions of different concentration, and varies with the temperature. 
If, however, yield value is plotted against temperature, the curve 
is again a straight line, which when produced cuts the temperature 










0-010 
. yee 
doce) anes 
ssigey HALLS 
HE 
Ls) 
E o. 
x aie al eR 
fees ee ee 
tonto 
Gj / 
PV MRE A a 
AGE RMmn mr ss 
0 300 600 $00 
Shearing Stress (Pressure). 
(Qa) 

& 

Aa 

8 

Q 

8 





0 W 20 30.40 50 60 70 80 90 
Yield Value. 
(b) 


Fic. 7 (Bingham and Hyden).—Graph (a) shows the relation between the shearing 
stress (pressure) and rate of flow through a capillary tube, of a dispersion of a certain 
sample of nitrocellulose in acetone. Concentration 7:-708%. Temperatures 5°, 
20°, 35°. The intercept on the pressure axis is friction or yield value f, which Bing- 
ham defines as the shearing stress at the wall of the capillary necessary to start the 
flow. Shearing stress is measured in dynes per square centimetre, and efflux in 
millilitres per second. Note that as the temperature rises, the yield value diminishes. 
Graph (6b) shows the yield value of the same solution plotted against the temperature. 
Note that the curve is linear, and on extrapolation cuts the temperature axis at 
about 43°, This behaviour suggests that this particular solution becomes a true 
liquid at 43°, since it is one characteristic Of pure liquids that they are deformed by 
any finite pressure, however small. At lower temperatures the flow of the dis- 
persion is partly a ‘‘ plastic flow,’’ characteristic of plastic solids. 


axis at a point which indicates the temperature at which yield value 
should be zero, i.e., at which the solution begins to behave like a 


true liquid (see Fig. 7). It was not definitely settled whether this 
temperature depends on the concentration. 


Cellulose Ester Solutions: Some Properties 81 


These observations are of the greatest interest to all chemists 
working with solutions of cellulose esters, and it is greatly to be 
desired that they may be continued and extended. f 

It is impossible in a paragraph to do justice to Bingham’s work, 
and the reader is referred to the bibliography at the end of the 
chapter for fuller accounts. His view that viscosities of colloid 
solutions as usually measured are not true viscosities does not 
invalidate the technical applications of such measurements under 
fixed conditions in the examination of cellulose esters, but he has 
already done much to distinguish and define the properties which 
give these solutions their special qualities, and his work may lead 
to a more rigid and scientific system for the definition and classi- 
fication of viscous colloids in general. Bingham’s interpretation 
of viscosity investigations has been vigorously criticised, e.g., by 
de Waele, who traverses the physical conceptions which Bingham 
has developed, and adduces interesting experimental evidence on 
the flow of petroleum jelly in tubes under low pressure. He also 
finds departure from linearity in pressure/outflow curves at greater 
ranges of pressure, and derives an empirical equation 


p=kit 
connecting pressure and outflow, ¢ being a constant less than unity. 
Bingham’s and Green’s reply to the criticism (ibid.) should also be 
read. It seems to the writer that since de Waele’s experiments show 
a marked difference in adhesion between the colloid and the wall of 
the capillary according to the pressure applied, a complete investiga- 
tion of the process must take account of the distribution of pressure 
in a semi-solid, which cannot take the same simple form as in a 
liquid. 
Tyndall Effect. 


A beam of light passing through an air space completely free 
from dust is invisible when looked at in a direction at right 
angles to the direction of the beam. The same is true of pure 
liquids when completely free from dust (“optically empty ”’). 
Liquids which contain particles in suspension, however, show the 
path of the light, even in many instances in which the particles 
are so small or so near in refractive index to the medium that the 
liquid appears transparent to the naked eye. This property is 
known from its chief investigator as the Tyndall effect, and has 
been applied largely to the study of colloidal solutions, particularly 
in ultramicroscopy. Solutions of cellulose esters show usually 
a weak Tyndall effect, because the swollen particles have a refractive 
index ed near to that of the medium in which they are suspended. 


82 Cellulose Ester Varnishes - 


Mardles *! examined the variation of Tyndall effect with concen- 
tration, using solutions of cellulose acetate, and made the 
interesting discovery that as the concentration of cellulose acetate 
increases, the effect rises to a maximum and then falls off (see 
Fig. 8). For example, solutions of a certain cellulose acetate in 
benzyl alcohol gave a marked maximum effect at a concentration 
of 6%. The peak to the curve is much less marked at higher 
temperatures than at low. The physical significance of this is 
uncertain. Since the effect is due to the difference between the 
refractive index of the particles and that of the solvent, when the 
effect increases there must be either an increase in the difference of 


dall Number. 


Tyr 
§ 





0.2 4 6 8,0 2 4 Biome ae 
Concentration in Grms. per 100c.c. 


Fic. 8 (Mardles).—These curves show the change of Tyndall number with con- 
centration for a sample of cellulose acetate in benzyl aleohol. Note that there is a 
very marked maximum at about 6% concentration, but this maximum is much less 
marked with rise of temperature. At 35° it has almost disappeared. The Tyndall 
numbers in which the scattering of light is measured are multiples of an arbitrary unit 
representing the scattering of light by a certain sample of castor oil used as a standard. 
The comparison was made photometrically. } 
refractive index or an increase in the amount of surface bounding 
the two phases, or both. The first condition would be brought 
about if the additions of cellulose acetate were taken up mainly 
by particles already existing; the second, either by the change 
in size or shape of the existing particles, or by formation of 
additional particles of swollen ester, or by subdivision of existing 
particles. The latter is so unlikely that it may be left out of 
account. If we assume the simplest behaviour of all, namely, 
that the cellulose acetate when dissolved in benzyl alcohol disperses 


into spheres of uniform size, the maximum surface will be exposed 


Cellulose Ester Solutions: Some Properties 88 


just before the boundaries of the spheres begin to touch, 7.e., when 
the spheres occupy 71% of the total volume of the solution. If 
more cellulose acetate is added, the boundaries will begin to 
coalesce, and the scattering of light to diminish. An explanation 
on these lines involves the assumption that in a 6% solution of 
cellulose acetate in benzyl alcohol the swollen particles occupy 71% 
of the total volume. A paper by Hatschek and Humphry 2 on 
agar sols and gels should be read in relation to this subject. It 
was pointed out by Porter in the discussion on the paper that the 
scattering of light need not indicate any great difference in com- 
position between the two phases, since it varies with the square 
of the relative refractive index. Hatschek agreed, but said that; 
on the other hand, it might require a considerable difference in 
composition between the two phases to cause even a slight alteration 
in the refractive index. 


REFERENCES AND BIBLIOGRAPHY. 


1 J. Masson and R. McCall, Trans. Chem. Soc., 1920, 117, 819-823. 
2 EK. C. Bingham, “ Fluidity and Plasticity.”” ° 8S. E. Sheppard, J. Ind. 
Eing. Chem., 1917, 9, 523. 4 W. H. Gibson and L. M. Jacobs, Trans. Chem. 
Soc., 1920, 117, 473-477. 5 E. W. J. Mardles, ibid., 1924, 125, 2244-2259. 
6 KE. W. J. Mardles, J. Soc. Chem. Ind., 1923, 42, 207-21llT. 7 E. W. J. 
Mardles, A. Moses and W. Willstrop, Advis. Comm. for Aeronautics, Rep 
No. 568, Dec. 1918. *® L. Meunier and A. Breguet, Rev. Gén. des Colloides, 
1924, 2, 289-294. °® G. Barr, J. Soc. Chem. Ind., 1924, 48, 1107. 2° J. N. 
Goldsmith, ibid., 1904, 28, 297 (it is possible that the fibres were originally 
invisible, owing to the refractive index being identical with that of the 
celluloid, and became visible later owing to a change in the refractive index 
of the latter brought about by loss of volatile solvent). 11 W. L. Balls, 
Proc. Roy. Soc., 1923, B, 95, 72. 1% J. Duclaux and E. Wollman, Bull. 
Soc. chim., 1920, 27, 414. 1° J. Boeseken, J. C. van den Berg, and A. H. : 
Kerstjens, Rec. trav. chim. Pays-Bas, 1916, 35, 320-345. 14 W. L. Barnett, 
J. Soc. Chem. Ind., 1921, 40, 61-637r. 15 J. C. Irvine and E. L. Hirst, Trans. 
Chem. Soc., 1923, 128, 518. 1° R. A. Punter, J. Soc. Chem. Ind., 1920, 39, 
3337. 17 H. de Mosenthal, zbid., 1904, 28, 295. 18 E. C. Worden, ‘‘ Nitro- 
cellulose Industry,” Vol. I., chap. iv. 1° F. Baker, Trans. Chem. Soc., 1913, 
103, 1653-1675. ?° H. Schwartz, Z. Chem. Ind. Koll., 1913, 12, 32. 21 C. 
Visser, Aeronaut. Research Comm., Memo. No. 758, Aug. 1920. ”% E. 
Hatschek, Z. Chem. Ind. Koll., 1912, 11, 284-286. #3 F. Baker, Trans. 
Chem. Soc., 1912, 101, 1409-1416. 4 E. Berl and R. Klaye, Zetisch. Schiess 
Sprengstoffe, 1907, 2, 381. 25 W. H. Gibson and R. McCall, J. Soc. Chem. 
Ind., 1920, 39, 1727, 2 A. Hi ghfield, Trans. Faraday Soe. .» 1921, 16, 94. 
a 35 I. O. Masson, ibid., 1921, 16, 95. ®8 I. N. Aas OR ti Rec. trav. chim. 
Pays-Bas., 1922, 44, 751-763. 29 G. Barr and L Bircumshaw, Advis. 
Comm. for Aeronautics, Rept. No. 663, Nov. 1919. *4 A. de Waele, J. 
Oil and Colour Ohem. Assoc., 1923, 4, 33-88. 30 FE. C. Bingham and W. L. 
Hyden, J. Franklin Inst., 1922, Dec., 731-740. 31 EK. W. J. Mardles, T'rans. 
Faraday Soc., 1923, 18, 318-326. %2 E. Hatschek and R. H. Humphry, ibid., 
1924, 20, 18-29. 

| Additional. References. 

E. Hatschek, (1) ‘“‘ Introduction to Physics and Chemistry of Colloids,” 
(2) Colloid Reports of British Association, No. 1, pp. 2—5 (a short report 
with a valuable bibliography). British Engineering Standards Association, 
** Determination of Viscosity in Absolute Units.” 


CHAPTER VI 
CELLULOSE ESTER SOLUTIONS: SOME PROPERTIES. PART II 


Swelling—Researches of Knoevenagel and his collaborators—Distribution of 
Solvent between Swollen Ester and Liquid—Volume Change on Dis- 
solution—Dielectric Capacity—Discussion of Evidence on Constitution 
of Cellulose Ester Solutions. 


Swelling. 


Brrore cellulose esters disperse to form what are usually termed 
solutions, they undergo swelling, evidently by absorption of one or 
more constituents of the solvent. They also undergo swelling in 
some liquids in which they do not disperse. This behaviour was 
investigated by the late E. Knoevenagel and his collaborators in a 
series of papers which probably rank with those of Mardles as being 
the most important of recent contributions to the study of the 
chemistry and physics of cellulose esters. 

The material employed was in all cases cellulose acetate in the 
form of threads resembling horse-hair. The most interesting experi- 
ments were those in which weighed quantities of this material 
were treated with different combinations of liquids, and the distribu- 
tion of the ingredients between the swollen ester and the supernatant 
liquid determined by analytical methods, in many instances specially 
devised. The results obtained can only be indicated very briefly 
here, and should be consulted in the original papers, which are worth 
republishing separately with the correction of some misprints. 

Cellulose acetate does not swell either in water or in absolute 
alcohol, but in mixtures of the two it swells considerably.1 The 
degree of swelling is determined by the composition of the mixture, 
and exhibits a maximum. It is not changed if, after equilibrium is 
reached, the organic portion of the solvent is entirely displaced by 
water. Swollen cellulose acetate is dyed by methylene-blue, or 
saponified by alkali, much more readily than the unswollen material, 
and these two processes run parallel with each other and with the 
degree of swelling. , 

When cellulose acetate ? is shaken with different aqueous solu- 
tions of aniline, phenol or ethyl tartrate, these ingredients are found 
after 24 hours to be distributed between the ester and the water 
according to the Distribution Law :— 


C4 = kCg 


where C'4 = the concentration of, say, aniline in the swollen acetate, 
C's = concentration of aniline in the liquid layer, and k = constant. 
This Law of Distribution is usually applicable to the equilibrium 


between two immiscible solutions of a common solute (e.g., to the 
84 


Cellulose Ester Solutions: Some Properties 85 


partition of picric acid between benzene and water in contact). 
Hence these experiments make it probable that when cellulose 
acetate swells, it takes up molecules as if it were a solvent of them, 
and not by adsorption at the surface. 

Measurements were next made? of swelling in the binary mix- 
tures benzene—alcohol, nitrobenzene—alcohol, carbon tetrachloride— 
alcohol. If the pure liquids were employed, the order of swelling, 
in decreasing values, was nitrobenzene, benzene, alcohol and carbon 
tetrachloride, of which only nitrobenzene could be called a powerful 
swelling agent. ‘The surface tensions of these pure liquids decrease 
in the following order—nitrobenzene, benzene, carbon tetrachloride, 
alcohol. In binary mixtures, alcohol always caused a lowering of 
surface tension, and a qualitative relation was iound between the 
degree of swelling and the lowering of surface tension. The greater 
the reduction of surface tension caused by the addition of alcohol, 
the greater the swelling action of the mixture. Since, however, 
alcohol alone has no swelling action on cellulose acetate, a maximum 
of swelling effect is reached at a certain concentration. 

A relation was then sought* between the degree of swelling 
(which precedes dispersion) and the viscosity of the solutions after 
dispersion. The viscosity—concentration curves were plotted for 
various mixtures of ethyl alcohol and nitrobenzene, and it was found 
that the viscosity curve for a liquid of weaker swelling power always 
lies below that for a liquid of stronger swelling power. An interesting 
regularity observed was that for any two mixtures of alcohol and 
nitrobenzene, the ratio of the viscosities at equal concentrations of 
cellulose acetate was constant. It was also found that the con- 
centration—viscosity data for any one solvent mixture were approxi- 
mately fitted by the equation 


concentration = constant x log. of viscosity. 


(The statement in the original paper, Koll. Chem. Beihefte, 1921, 
13, 267, “ Die Viskositat steigt im gleichen bindiren Lésungsgemisch 
bei zunehmenden Azetylzellulosegehalt annahernd proportional mit 
den Logarithmen der inneren Reibung an,”’ is an evident slip.) 
Further investigations into the system cellulose acetate—nitro- 
benzene-alcohol > confirmed previous work by showing that the 
nitrobenzene over a range of concentrations obeyed the Distribution 
Law (Henry’s Law), i.e., there was a constant ratio between the 
concentration of nitrobenzene in the liquid and in the swollen solid. 
It was also found that alcohol was taken up in constant amount by 
the cellulose acetate through a considerable range of mixtures 


\ 


86 Cellulose Ester Varnishes 


containing small concentrations of nitrobenzene. The experiments 
were extended ® to the mixtures acetic acid—water, acetic acid— 
benzene, acetone—-benzene, nitrobenzene-isopropyl alcohol, ethyl 
alcohol—benzene, nitrobenzene—benzene, ethyl alcohol—water, acetic 
acid—nitrobenzene, acetone—nitrobenzene, methyl alcohol-nitro- 
benzene, acetone—methyl alcohol, acetic acid-camphor. It now 
begins to be difficult to follow the deductions which Knoevenagel 
made from the experimental results. Consider, for example, the 
system cellulose acetate—benzene-acetone. It is stated that there is 
a constant ratio of cellulose acetate to benzene in the swollen acetate ; 
also that both benzene and acetone are distributed between the 
liquid and the swollen acetate according to Henry’s Law. Using 
his nomenclature, we have :— 
mols. of acetone per cent. = a. 
mols. of benzene per cent. = b. 

mols. of acetone per cent. = c. 
In the swollen acetate jmol of benzene per cent. = d. 

mols. of cellulose acetate per cent. = e. 
(assuming a C, unit of cellulose acetate). 


In the liquid { 


It is stated (p. 186) that © = constant (mean value 1-05, range 0-91 
to 1-18). 

» (mean value 1-11, range 0-85 
to 1-26). 

» (mean value 5-09, range 4-31 


to 7:42). 
If these three relations are all true, it follows that 


| 


| 


QO} Qi Qo 


b 
: = constant. 


Actually : varies from 8-8 to 1-7, so that it appears that the con- 


stancy of one or more of the three ratios mentioned above has been 
too readily assumed. It is greatly to be desired that this matter 
should be cleared up, preferably by some of Knoevenagel’s collabora- 
tors, as the experimental work forming the basis of this series of 
researches is invaluable. It is right to add that Knoevenagel 
admitted the existence of deviations in this series, probably owing 
to the chemical relations between acetone and benzene, and the 
solubility of cellulose acetate in acetone. 

His general conclusion is that alcohol, benzene and water are 
taken up in constant quantities from various mixtures, and this 


an ee 


Cellulose Ester Solutions: Some Properties 87 


suggests that swelling is due to chemical action and not to surface 
adsorption. The affinities concerned are probably of a different 
order from those concerned in typical molecular compounds. 

In the next paper,’ similar experiments are described, from which 
it is concluded that from binary mixtures of alcohol, nitrobenzene 
and benzene, liquid is absorbed by cellulose acetate in molecular 
proportions. The sum of the number of molecules of each liquid 
taken up from a binary mixture by cellulose acetate is constant. 
Determinations of heat change support the view that swelling is a 
molecular process. 

In the last publication, ® it is shown that cellulose acetate which has 
been brought into equilibrium with a solvent in which it swells 
considerably, will, if placed in a solvent with smaller swelling power, 
shrink until it possesses the same degree of swelling as it would have 
had if it had been placed originally in the second solvent. This is 
in contrast with the behaviour noticed with water, which can entirely 
replace an aqueous—organic swelling mixture such as alcohol and 
water, without changing the degree of swelling. Probably the water, 
which has no swelling action whatever, hardens the original swollen 
structure and sets it. 


Volume Change on Dissolution. 


Mardles ® found that when cellulose acetate is dispersed in a 
solvent, a slight contraction in volume takes place. With simple 
solvents, the greatest contraction takes place with the best solvents. 
With mixed solvents, the contraction is greatest with the mixture 
of highest solvent power. It is proportionately greater at low 
concentrations and at high temperatures, 7.e., under those conditions 
in which the dispersion approaches most closely to a molecular 
solution. 

It should be noted that these observations refer to the system 
as a whole, whereas the “ swelling’ studied by Knoevenagel and 
his collaborators refers only to the colloid. Thus the colloid swells 
(by absorption of solvent molecules), but the system as a whole 
contracts. Further, Knoevenagel found that solvent mixtures in 
which the swelling was small yielded solutions of lower viscosity 
than those in which swelling was great. It is generally agreed now 
that solutions of low viscosity indicate high solvent power, and it 
would be expected that when the swelling of the colloid was least, 
the contraction of the whole system would be greatest. Hence we 
see a qualitative connection between low swelling of the colloid, 
high contraction of the system, and low viscosity of the solution. 


88 Cellulose Ester Varnishes 


Dielectric Capacity. 


Fenton and Berry 1° thought from the investigation of the solvent 
power of a large number of simple liquids for cellulose acetate that 
there was some relation between the dielectric constant and solvent 
action, although there were admittedly exceptions. Mardles 14 
quotes a number of these exceptions and points out that the dielectric 
constant diminishes with rise of temperature, whereas solvent 
action increases. 4 


Conclusion. 


The preceding discussion on some of the properties of solutions 
of cellulose esters which are most nearly related to their industrial 
application, furnishes material for provisional views of their nature. 

The dispersion of a cellulose ester by a simple or mixed solvent 
obviously depends in some way on the attraction exerted by the 
molecules of the solvent, or some groups in the molecules, on some 
of the distinctive groups in the esterified cellulose. Nevertheless, 
it must always be remembered that any picture based on this con- 
sideration alone will be incomplete, since on this view cellulose, with 
its high percentage of hydroxyl groups, should be soluble in water. 
The secondary affinities, which bind together the Cj, units of cellulose 
proposed by Irvine, still exist in the cellulose ester, although probably 
they are weakened. 

Ksselen 1 was apparently the first to suggest the application of 
Langmuir’s theory of film adsorption to the dispersion of cellulose 
_esters. Considering the example of cellulose acetate dispersing in 
certain mixtures of chloroform and alcohol, he assumes that the 
surface of cellulose acetate attracts the hydroxyl groups of the 
alcohol and adsorbs the alcohol in such a way as to hold the hydroxyl 
groups next to the acetate, leaving the hydrocarbon (ethyl) radical 
projecting into the solution. These hydrocarbon groups may 
attract the hydrocarbon end of the chloroform molecules. Both of 
these processes would induce swelling and tend to disperse the 
cellulose acetate. Esselen further suggests that the composition 
of the best solvent mixture may coincide with the point at which all 
the secondary valencies at the surface of the acetate are just saturated 
by the alcohol. With technical cellulose acetate this point corre- 
sponds with 25 to 30% alcohol in the solvent. 

As a second example, Esselen considers the solubility of some 
varieties of cellulose acetate in a warm mixture of alcohol and 
benzene, although they are soluble in neither constituent alone nor 
in the mixture when cold. In this instance he supposes a similar 


Cellulose Ester Solutions: Some Properties 89 


adsorption of alcohol by the ester, and an attraction between the 
ethyl radical of the alcohol and the benzene hydrocarbon. That a 
paraffin hydrocarbon does not behave similarly he explains by the 
known fact that the paraffin hydrocarbons and alcohol are only very 
slightly miscible, so that if the ester only dissolves in the alcohol— 
benzene mixture with difficulty, one would not expect the alcohol- 
paraffin mixture to possess any solvent power at all. 

Esselen then extends the theory to compounds possessing the 
aromatic nucleus and the hydroxyl group in one molecule and 
suggests that this may explain the high solvent power of the phenols. 
[Perhaps a better example still would be benzyl alcohol. ] 

This theory appears to contain the germ of a rational mode of 
viewing the dispersion of cellulose esters, and it is worth examining 
in the light of the evidence summarised in previous pages. If the 
best solvent mixture corresponds to a point at which the surface 
is entirely saturated with alcohol, the composition of the best solvent 
mixture should vary with the concentration of the ester. On this 
point there is conflict of evidence. Highfield states that, in the 
instance of cellulose nitrate and ether—alcohol, the composition of the 
best mixture is independent of the concentration, while Masson and 
McCall, working with acetone and water, found slight variations 
with the concentration. It does not seem to be necessary to assume 
that the whole of the alcohol in the best solvent mixture is adsorbed 
at the surface of the ester and it is not clear that this is Esselen’s 
assumption. -Itis more reasonable to assume an equilibrium between 
the alcohol so adsorbed and the alcohol in the surrounding medium. 
A change in the concentration of the ester would displace this 
equilibrium, but would not lead to a quantitative adsorption of more 
ethyl alcohol exactly proportionate to the fresh surface of ester 
supplied. Another factor which may enter here is that the swelling 
of the ester may itself open up fresh surface to the adsorption of 
alcohol, and if this is inhibited in any way at higher concentrations, 
the amount of surface will not be proportional to the concentration. 

The dispersion of the ester would therefore be.due to the affinity 
of one or more constituents of the solvent mixture for groups on the 
surface of the particle of cellulose ester, some of these groups being 
formed by the particle selectively adsorbing constituents from the 
solvent mixture itself. Hence, in a restricted sense, the formation 
of a complex between the constituents of a binary mixture may be 
related to its solvent power, provided that one constituent of the 
complex is also selectively adsorbed by the particles of cellulose 
ester. This constituent acts as a link between the cellulose ester 


90 Cellulose Ester Varnishes 


and the solvent medium, and through it the molecular motion of 
the latter is transmitted to the ester as a dispersive influence. Since 
vapour pressure is a manifestation of molecular energy, we see why 
substances of low boiling point are more active constituents of solvent 
mixtures than their homologues of higher boiling point, and it is 
interesting that chloroform and ether, which when mixed with 
alcohol form solvents for cellulose acetate and nitrate respectively, 
are both liquids of high vapour pressure. Possibly this property is 
also a factor in the use of the volatile substance carbon di-sulphide 
in the viscose reaction. 

Single solvents must be regarded as combining in one substance 
the function of being adsorbed by the cellulose ester, and of being 
attracted also by the molecules of their own kind in the remainder 
of the solvent medium. Thus, the solvent action of benzyl alcohol 
on cellulose acetate would be explained by the selective adsorption 
of the hydroxyl group by the particles of cellulose acetate, and 
the attraction between the remainder of the molecule, namely, 
C,H;°CH,°, and the rest of the benzyl alcohol in the solvent. Such an 
attraction would be expected to manifest itself in a certain degree 
of association in the pure solvent, and we have a speculative 
explanation of the fact that associated liquids are frequently good 
solvents. 

The increased solvent power frequently imparted by small 
quantities of water in solvents may be due to a similar attraction 
between hydroxyl groups in the ester and water molecules, and the 
complex so formed may then adsorb some water-soluble constituent 
of the rest of the solvent. Worden } states that if the acid hydrolysis 
of cellulose acetate is unduly prolonged, the resulting product will 
take more and more water in its solution in acetone without precipita- 
tion. There is an undoubted relation between these factors, and 
also a possibility that the ester groups themselves may have an 
inherent attraction for water molecules, since all the inorganic 
nitrates are soluble in water, and most of the acetates. 

It is not profitable, however, in the present state of our knowledge 
to push the theory of selective solvent absorption or adsorption 
too far. There are specific influences at work also, which must 
explain why alcohol and chloroform disperse cellulose acetate and 
not cellulose nitrate, while ether and alcohol disperse cellulose 
nitrate and not cellulose acetate; in each case we certainly have 
adsorption of alcohol and some affinity between the alcohol and the 
other ingredient; yet entirely different behaviour. Perhaps the 
underlying attraction of the theory of selective absorption is that it 


Cellulose Ester Solutions: Some Properties 91 


suggests an analogy with selective permeability of membranes, and 
a possibility of applying a modification of Donnan’s theory of 
membrane equilibria to the swelling of cellulose esters, thus bringing 
them into line with gelatin. 

Against the suggestion that the dispersion of cellulose esters in 
a solvent may be brought about by selective adsorption of constituent 
groups of the solvent mixture at the surface of the ester particles, 
must also be placed the weight of evidence adduced by Knoevenagel. 
If solvent molecules are absorbed by the ester so that the ratio of 
_ the number of molecules to the number of C, units is constant, 
Knoevenagel appears to be justified in assuming the formation of 
some kind of molecular compound, and in asking the question—What 
has become of the surface? Perhaps, after all, this is only a restate- 
ment of an old difficulty. What structure is it that prevents 
cellulose dissolving in water and yet allows the penetration of a 
nitrating acid mixture (presumably) to every C, unit in the mass ? 
A great deal more experimental evidence is required, particularly 
along the lines indicated by Knoevenagel, before these questions 
can be answered. 

In regard to the molecular dimensions of the particles in dis- 
persions of cellulose esters, there is also contradictory evidence. 
Béeseken and his collaborators think that there is not a great 
range of dimensions, 7.e., that » does not vary much in the 
particles of cellulose acetate when acetylation and the accompanying 
hydrolysis are not pushed too far. Duclaux and Wollman, on the 
other hand, find a wide range of viscosities in the products of the 
fractional precipitation of cellulose nitrate, and the same fact holds 
for cellulose acetate. Since nitration is a shorter process than 
acetylation, one would expect the range of dimensions of cellulose 
nitrate to be at any rate no greater than that of cellulose acetate. 
One way of reconciling the two views is to assume that a com- 
paratively small difference in the average value of n causes a large 
difference in the viscosity of the solutions. 

The peak in the curve connecting Tyndall effect and concentra- 
tion (Mardles) requires much more investigation, using different 
esters and different solvents. Since the cellulose esters usually 
differ in refractive index from their solvents, a dispersion of the ester 
in the solvent would be expected to show a Tyndall cone, as long as 
the particles were not too small. If, however, the particles swell 
very considerably by adsorption of solvent, the difference in refrac- 
tion between the two phases must diminish and the Tyndall cone 
become less marked, unless we assume that orientated benzyl 


92 Cellulose Ester Varnishes 


alcohol molecules, 2.e., as produced by a shell of C,H,;-CH,* groups 
forming the outside of a cellulose acetate particle which has selec- 
tively adsorbed the ‘OH groups, have a different refractive index 
from normal benzyl alcohol. It has already been suggested that the 
peak may represent the maximum development of surface before 
envelopes of solvent, surrounding ester particles, begin to coalesce. 
If so, there should be a relation between this curve and the curve 
connecting concentration with yield value (Bingham). An investiga- 
tion on these lines would be more likely to yield results of value if 
the particles were of approximately uniform size, and this condition 
could be partly realised by using the products of a fractional precipi- 
tation (Duclaux) as working material. Further, Mardles found that 
the peak diminished and disappeared with rise of temperature, while 
Bingham (with a different ester and solvent) found that solutions 
showing zero fluidity approached and finally attained the status of 
true liquids with rise of temperature. A relation should be sought 
between these phenomena, to see whether the temperature at which 
a cellulose ester dispersion becomes a true solution is coincident with, 
or related to, the temperature at which the peak in the Tyndall- 
concentration curve disappears. 

This short survey of recent researches on the properties of cellu- 
lose ester solutions is enough to show that much remains to be done, 
and a great deal of the work can best be carried out in academic 
research laboratories. Many of the measurements required, though 
comparatively simple and involving no great manipulative skill, 
are tedious and require for their performance a degree of freedom from 
the claims of other interests which can seldom be realised in a factory 
laboratory. Apart from their scientific interest, there is always the 
possibility of discovering some fact of immediate technical value. 


REFERENCES. 


1 KH. Knoevenagel and O. Eberstadt, Koll. Chem. Beihefte, 1921, 18, 194- 
212. 2? E. Knoevenagel and R. Motz, ibid., 1921, 13, 233-241. 3 'E. Knoe- 
venagel and A. Bregenzer, <zbid., 1921, 13, 249-261, 4 Idem, abid., 1921, 
18, 262-271. 5 Idem, ibid., 1921, 14, 1-24, ° KE. Knoevenagel, J. Hogrefe, 
and F. Mertens, zbid., 1922, 16, 180-214. * E. Knoevenagel and E. Volz, 
tbid., 1923, 17, 51-71. ® E. Knoevenagel, abid., 1923, 18, 39-43. ® E. W. J. 
Mardles, Trans. Faraday Soc., 1923, 18, 365-385. 1° H, J. H. Fenton and 
A. J. Berry, Proc. Camb. Phil. Soc., 1920, p. 16. 11 E. W. J. Mardles, J. 
Soc. Chem. Ind., 1923, 42, 127-1367. 1% G. J. Esselen, jun., J. Ind. Eng. 
Chem., 1920, 12, 801-803. 18 E. C. Worden, J. Soc. Chem. Ind., 1919, 38, 
370-3747. 


CHAPTER VII 


INGREDIENTS OF CELLULOSE ESTER VARNISHES 


Inflammability of Solvents—Inflammability of Coatings—Choice of Solvents 
and Diluents—Application of Laboratory Results—Variation in the 
Esters—Commercial Solvents of Esters—Approximate Classification— 
Specifications for Acetone, Methyl Ethyl Ketone, Methyl Acetone, Ethyl 
Alcohol, Methyl Alcohol and Wood Spirit, Ether, Ethyl Acetate, Butyl 
Acetate, Amyl Acetate, Ethyl Lactate, Acetone Oil, Diacetone Alcohol, 
Tetrachloroethane, Benzene, Toluene, Butyl Alcohol, Amyl Alcohol, 
Triacetin, Benzyl Alcohol, Triphenyl Phosphate, Castor Oil—Resins— 
Shellac, Mastic, Copal, Sandarac, Dammar—Typical Resin Formule. 


THE processes and considerations involved in the manufacture 
and application of cellulose nitrate and cellulose acetate varnishes 
are so similar that it is best to treat them together. The choice 
between the two is mainly that of price, but occasionally other 
considerations occur, such as the inflammability of the coating or 
the resistance of the coating to particular solvents. 

In regard to inflammability, misapprehensions have frequently 
arisen as to the comparative dangers of the two esters. Dry cellulose 
acetate itself, the base of the acetate varnishes, is usually termed 
non-inflammable—generally as a distinction from cellulose nitrate— 
because although it will burn (7.e., is combustible), it burns quietly 
and presents no special fire danger. Dry cellulose nitrate, on the 
other hand, burns fiercely, and in the higher nitrations is explosive 
under certain conditions. The solutions of these esters which are 
employed as varnishes, however, invariably contain volatile inflam- 
mable solvents or diluents such as acetone, methyl alcohol, ethyl 
alcohol, benzene and ethyl acetate, to which they owe their property 
of drying quickly. The danger incurred in the application of these 
varnishes is therefore practically equal for both esters, since it 
arises only from the accumulation of inflammable vapours in the 
working rooms, unless simple precautions are taken in regard to 
ventilation. These inflammable vapours in certain ranges of 
concentration form explosive mixtures with air, and even when all 
precautions are taken, naked lights, fires or smoking should never 
be allowed in the work-rooms. Exactly the same precautions are 
' necessary with the so-called spirit varnishes, which consist chiefly 
of resins dissolved in methylated spirit. The danger during appli- 
cation has nothing to do with the solid materials dissolved in the 
solvents; it arises from the properties of the solvents alone. 

When the volatile solvents have dried out, leaving the film of 
cellulose ester, with or without softeners, resins or pigments, a new 
set of conditions arises. ‘The film of cellulose nitrate, taken alone 


and away from any support, is still highly inflammable. If, however, 
93 


94, Cellulose Ester Varnishes 


the film is attached to a support—as in practice it always is—the 
danger due to its inflammability in most cases disappears. On a 
metal surface, the film cannot be made to ignite, as the application 
of a flame only causes it to blister and decompose without flaming. 
Wood coated with the varnish is no more easy to ignite than uncoated 
wood. In fact, one may say that in general the fire danger of 
materials coated with a cellulose nitrate varnish is the same as that 
of the materials alone. 

There is, however, one important class of exceptions—that of 
textiles covered with a thick protective coating of cellulose ester. 
In this instance we have an inflammable fabric, together with a 
coating which forms an appreciable fraction of the total weight of the 
coated material. Here a cellulose nitrate coating is an added 
danger, and it is from this fact that the use of cellulose acetate 
varnishes for aeroplane fabrics has become so extensive. Even 
cellulose acetate varnishes for this purpose often have special 
ingredients added so as to reduce their non-inflammability to incom- 
bustibility, since the film from a varnish not so treated is usually 
appreciably more inflammable than the original cellulose acetate 
base, on account of the inflammable organic softeners and solvents 
of high boiling point which it contains. 


Choice of Solvents. 


In nearly all technical varnishes based on cellulose esters, the 
solvent is a mixture of several liquids. The reason for this is obvious. 
The chief advantage possessed by these varnishes over oil varnishes 
is their rapid rate of drying, due to the volatility of their solvents. 
If, however, a varnish is made up entirely from a solvent of low 
volatility, e.g., acetone, it is found that the rapid rate of drying 
entails such a marked fall in temperature at the surface of the film 
that the atmosphere begins to deposit moisture upon it, causing 
precipitation of the ester and sometimes pitting of the film. This 
tendency imposes a practical limit on the rapidity of drying which 
can be allowed in a commercial varnish and will be considered in 
more detail in a later section. 

The varnish must therefore contain only a limited quantity of 
highly volatile solvents if it is to dry in a normal atmosphere with a 
bright surface and a coherent structure. To lower the rate of 
evaporation we must add a solvent of higher boiling point, and quite 
satisfactory varnishes for many purposes can be made from a simple 
mixture of solvents with boiling points varying from low (e.g., about 
50°) to high (about 130°). Here, however, considerations of cost 


Se ys 


Ingredients of Cellulose Ester Varnishes 95 


enter. Solvents of the cellulose esters are usually comparatively 
expensive materials, and we have to find in what way we may dilute 
these expensive ingredients with cheaper materials without inter- 
fering with their properties as varnishes. These diluents must, of 
course, be miscible with the solvents used. They must not precipi- 
tate the cellulose ester from the solution, and this condition must 
hold not only in the original varnish, but at all stages of the evapora- 
tion of the volatile ingredients. For example, a solution of cellulose 
nitrate in acetone could easily be diluted with a considerable quantity 
of xylene without precipitating the ester. If, however, such a 
solution were poured on to a glass plate and allowed to evaporate, 
even in a perfectly dry atmosphere so that no deposition of moisture 
was to be feared, it would not form a satisfactory film. The acetone 
would evaporate much more rapidly than the xylene, and the point 
would soon be reached at which the cellulose nitrate would begin 
to separate from the solution. The final product would be an 
opaque, flaky and brittle deposit of no value. 

If, however, a diluent is chosen having a suitable rate of evapora- 
tion, it is possible to make use of it. For example, a solution of 
cellulose nitrate in amyl acetate may be considerably diluted with a 
light petroleum spirit, e.g., one which distils between 50° and 120°, 
without precipitating the ester, and during the evaporation the amyl 
acetate may be continuously present in sufficient quantity to keep 
the ester in solution. Under these circumstances a clear tough film 
is formed. 

The preceding chapters have shown, however, that there are 
other cheap diluents which may be used in combinations which 
actually assist the true solvents. Thus alcohol has been shown to 
reduce the viscosity of solutions of both cellulose acetate and 
cellulose nitrate, when used in certain combinations. For example, 
alcohol and toluene together have a solvent action on some grades 
of cellulose nitrate, and alcohol and benzene a weaker solvent action 
on cellulose acetate, and it is for this reason that the laboratory 
researches on the solvent power of these mixed liquids are so impor- 
tant. Since alcohol and the coal tar distillates are cheaper than 
most of the true solvents, the use of certain combinations of them as 
diluents of the true solvents will often achieve considerable economy 
without detriment to the varnish. 


Application of Laboratory Results. 
These combinations must, of course, be chosen with considerable 
care, and much experience is necessary in applying the results of 
laboratory work to technical manufacture. Apart from the 


96 Cellulose Ester Varnishes 


necessity of avoiding excessively rapid evaporation and the precipi- 
tation of the ester at any stage of evaporation, three other considera- 
tions enter :— 


(1) Most laboratory investigations are rightly carried out with 
chemically pure solvents. Technical solvents are seldom 
chemically pure and in some cases the degree of impurity may be 
somewhat large. The influence of these impurities is irregular. 
Often it may be favourable, as in the presence of traces of 
acetone in methyl alcohol (as a cellulose nitrate solvent), or of 
acetone in methyl ethyl ketone (as a cellulose acetate solvent). 
Whether favourable or unfavourable, however, the presence of 
these impurities complicates the direct sae of the 
results of research. 

(2) When a solvent contains three or more ingredients 
(and in practice simpler mixtures are rare) the application of the 
results of the investigation of binary mixtures becomes very 
complicated, even if the ingredients are chemically pure. Take 
for instance a mixture of amyl acetate, ethyl alcohol, benzene 
and acetone. It could be regarded as a mixture of two binary 
mixtures, (1) amyl acetate + ethyl alcohol, (2) benzene + 
acetone, and the solvent properties of these separate mixtures 
might be known, but it would be impossible to forecast the 
solvent properties of the complete mixture by making use of 
this knowledge alone. Each one of these ingredients gives 
characteristic viscosity and solvent power curves with each of 
the other three, and the mutual interaction of all these mani- 
festations of affinity cannot be accurately gauged. 

(3) A point which is repeatedly overlooked in patent and 
other literature is that no two samples of cellulose ester can be 
guaranteed to be exactly alike. Profound differences are possible 
in solubility, in viscosity and in chemical composition. Even 
with the most careful control of manufacture small variations 
occur in cellulose esters made by the same process and in the 
same factory, and between one manufacturer’s product and 
another there may be considerable differences due to variations 
in the raw material, composition of the esterifying mixture and 
the time and temperature of esterification. . 

It follows from the last paragraph that the value of long lists of 
formule of cellulose ester varnishes is doubtful, especially since 
the advent of cellulose nitrates of unusually low viscosity. For 
instance, two separate samples of cellulose nitrate varnish might 


Ingredients of Cellulose Ester Varnishes 97 


be made up to exactly the same formula. One might yield a 
stiff jelly which would not flow appreciably when the vessel 
containing it was inverted. The other might give a solution 
more fluid than glycerine. The chief interest and value of 
published formule lie in the manner of combination of the 
solvents. 


In a recent circular, Gardner and Parkes! discuss the rate of 
evaporation of solvents from pyroxylin lacquers, on the whole 
along well-known lines; there are, however, some points of interest. 
Too rapid evaporation of the solvent causes the faults variously 
described as pin-holing, goose-fleshing and the orange-peel effect. 
Tests on the rate of evaporation of individual solvents alone, at 
ordinary temperatures, show that the order in which the solvents 
are placed by this test does not exactly coincide with the order of 
the boiling points; for example, ethyl alcohol evaporates into the 
air more slowly than ethyl! acetate, although the boiling points are 
almost identical. This was to be expected, since the rate of evapora- 
tion at ordinary temperatures depends on the vapour pressure at 
that temperature, and the vapour pressure curves of different 
substance may cross one another; the boiling point is only a rough, 
but nevertheless useful, indication of the rate of evaporation at 
ordinary temperatures. Curves are given for the rate of drying of 
solutions of nitrocellulose in ethyl acetate, denatured alcohol (using 
a special alcohol-soluble nitrocellulose), diethyl carbonate, butyl 
acetate and amyl acetate. The time taken for 87% of the solvent 
to evaporate is given in the following table :— 


Ethyl acetate. ; : ‘ : . 380 mins. 
Ethyl alcohol ; ; . : ey ne Oe 
Diethyl | 110 
Butyl acetate . ie 
Amyl acetate . ; ; : : Been bei" ae 


The test with ethyl alcohol is hardly comparable with the others, 
since it was carried out with a special nitrocellulose, and the nature 
of the dissolved ester undoubtedly influences the rate of evaporation. 
The other four tests were carried out with the same sample of 
nitrocellulose. 

Curves are also given showing the rate of evaporation of butyl 
acetate from solutions of different concentration of the same nitro- 
cellulose. These all appear to reach approximately constant weight 
in 120 minutes on filming, although the amount of solvent left in the 
films shi from 3% for the thinnest film to 26% for the thickest 


98 Cellulose Ester Varnishes 


film. Closer investigation would, no doubt, show that the thicker 
films were still losing solvent at a slow rate. ‘The writer’s experience 
in the celluloid industry is that there is a rapid loss of solvent from 
unseasoned material in the first few hours which is practically the 
same per unit of surface whether the material is thick or thin. ‘The 
rate quickly diminishes, but thick material will continue to lose 
weight long after thin material is practically dry. 


Commercial Solvents for Cellulose Nitrate. 


The principal solvents in use for cellulose nitrate varnishes are 
acetone, methyl alcohol, ethyl acetate, butyl acetate and amyl 
acetate. Of these, ethyl acetate is used more in the United States 
than in this country. Acetone and methyl alcohol are both highly 
volatile at ordinary temperatures, and solutions in either of these 
solvents will not give clear films unless special precautions are taken 
to reduce the humidity of the air and the rate of evaporation. Butyl 
acetate and amyl acetate are used in nearly all the technical var- 
nishes, as, owing to their low vapour pressure, they retard the rate 
of evaporation and diminish the tendency of the film to absorb 
moisture from the atmosphere. They can be used with diluents 
having a vapour pressure only slightly less than their own, as long 
as they are present in sufficient amount throughout the evaporation 
to prevent separation of the cellulose nitrate. According to Wiesel,? 
diethyl carbonate is being introduced as a solvent in the United 
States. Cyclohexanone and cyclohexanyl acetate have also been 
suggested, but have not yet come into general use. The chief 
diluents are ethyl alcohol, benzene, toluene, butyl alcohol and amyl 
alcohol (or fusel oil). Ethyl alcohol is present in practically all 
cellulose nitrate varnishes, since it is used to expel water from the 
original cellulose nitrate in the dehydration process. It is not 
simply a diluent, since it develops the solvent power of some other 
solvents. Benzene and toluene are both coal-tar fractions. They 
have no solvent power of their own, but are partial or complete 
solvents when mixed with alcohol in certain proportions. When 
toluene, which has a lower vapour pressure than benzene, is used, 
special care must be taken that there is sufficient butyl or amyl 
acetate present to counteract its lack of solvent power when the 
bulk of the alcohol has evaporated. 

Finally, it may occasionally be economical to dilute a solution 
with an entirely indifferent liquid which has no solvent power, either 
alone or in combination, at ordinary temperatures. The only 
liquid of this class used commercially is a light petroleum spirit. 


| 
| 


Ingredients of Cellulose Ester Varnishes 99 


The heavier paraffin distillates are too slow in evaporation, and 
would not satisfy the condition that the true solvents must be in 
excess throughout the evaporation. 


Cellulose Acetate Solvents. 


These may be considered under practically the same headings as 
cellulose nitrate solvents. Clément and Riviére® classify the 
acetate solvents as follows :— 


(1) Direct volatile solvents : Acetone, methyl acetate, methyl 
and ethyl formates, diacetone—-alcohol, nitromethane, formic 
and acetic acids, pyridine, ethyl acetoacetate, benzyl alcohol, 
furfurol. 

(2) Indirect volatile solvents: Hot alcohol and benzene, 
chloroform and alcohol, tetrachloroethane and alcohol. 

(3) High-boiling gelatinising solvents: Acetins, particularly 
triacetin, diphenyl ethers of glycerin, glycerin benzoate, glycol— 
chlorohydrin, resorcin diacetate, eugenol, butyl and amyl 
tartrates, dimethyl phthalate. 


One may say roughly that, in the above list, acetone, methyl acetate 
and methyl and ethyl formates correspond to the highly volatile 
sc ~°nts used in cellulose nitrate varnishes. To these may be 
addea methyl ethyl ketone, which, although not a solvent when 
chemically pure,* always contains, in its technical form, sufficient 
acetone to make it one. Benzyl alcohol, eugenol and the acetins 
are similar in function to butyl and amyl acetates in cellulose nitrate 
solutions; while alcohol and benzene, alcohol and chloroform, 
_ alcohol and tetrachloroethane are binary mixtures possessing definite 
solvent power and not to be classified as mere diluents. Entirely 
indifferent diluents such as petroleum spirit do not appear ever to 
be used in cellulose acetate solutions. ‘The solvents used in the Air 
Ministry dopes are acetone, methyl ethyl ketone, alcohol, benzene 
and benzyl alcohol. Many of the additional substances in Clément 
and Riviére’s list were used to a greater or less extent during the war, 
but have been entirely displaced by cheaper solvents since. 


Specifications of Cellulose Nitrate and Acetate Solvents. 


Specifications for all of the usual solvents have been published 
by the British Engineering Standards Association in accordance 
with the best commercial practice. They are copyright, and can be 
obtained from the Secretary, 28 Victoria Street, London, 8.W.1, at. 
2d. each, post free. 


100 Cellulose Ester Varnishes 


These specifications refer generally to materials required for 
Air Ministry Dopes, and in most cases it may be advisable to make 
them more or less stringent for other commercial purposes. On 
the whole, however, they form a reasonable basis for the examination 
of solvents. 

In general, it may be said that a high standard of chemical purity 
is not necessary for the ingredients used in the preparation of 
cellulose ester varnish, since it follows from what has been written 
before that the solvent power of mixtures is often greater than that 
of either ingredient singly; but it is clearly necessary for the buyer 
to know what he is buying, and to have the mixing under his control. 
Further, there are certain classes of impurities which must be 
stringently guarded against. One of these is acidity. Both the 
acetate and the nitrate of cellulose are sensitive to strong acids, and 
even weak acids such as acetic acid may be harmful in their effects 
on containers, pigments, or fabrics. A second harmful impurity is 
water. Although water in small amounts increases the solvent power 
of some solvents, in large amounts it acts as a precipitant and 
destroys the tenacity of the coating. It must also be remembered 
that considerable cooling takes place when a cellulose ester coating 
is drying, and this always results in a certain amount of moisture 
being absorbed from the atmosphere, even if the original ingredients 
were anhydrous. A third undesirable impurity is solid foreign 
matter, either suspended or dissolved in the solvent. Suspended 
solids can be removed by filtration, but dissolved solids will be left 
in the varnish coating. 

Hence specifications of cellulose ester solvents usually contain 
the following clauses: Water content, acidity and residue on 
evaporation are strictly limited. Other impurities are limited by 
determinations of one or more physical properties such as specific 
eravity, range of temperature of distillation, refractive index, 
miscibility with specified liquids or solutions, and colour. Usually 
also, if practicable and convenient, there is a chemical determination 
of the percentage of the pure chemical compound in the sample. 


Specifications for Cellulose Nitrate and Cellulose Acetate Solvents. 
The following are typical specifications for some of the commoner 
solvents and diluents used in the cellulose ester varnishes :— 


Acetone.—Until the introduction of the new cordite (R.D.B.) 
during the war, acetone was used principally as a solvent for gun- 
cotton, and the best quality was supplied to the British Government 





Ingredients of Cellulose Ester Varnishes 101 


Specification (usually abbreviated B.G.S.). This specification is 
still used almost universally in commerce. 

B.G.8. acetone must be colourless and transparent, and must 
mix with distilled water in any proportions without showing tur- 
bidity. It must leave no residue when evaporated to dryness. Its 
specific gravity at 15°/15° must not exceed 0-800. If 100c.c. of the 
acetone is mixed with 1 c.c. of a 0-1% solution of potassium perman- 
ganate the colour must not be discharged in less than 30 minutes 
in the dark at a temperature of 15°. This last test, known as the 
permanganate test, is the one on which acetone is most frequently 
rejected. In 1904, as a result of researches by Marshall,' a clause 
was added to the specification to ensure that the acetone should 
not contain more than 0-002% of carbon dioxide, and should be 
otherwise quite neutral. Carbon dioxide has no harmful effect in the 
manufacture of lacquers, and it is usual only to limit the percentage 
of acidity and basicity (v. infra). 

A useful additional test is the amount of discoloration on exposure 
to light. This is important if the acetone is to be used in colourless 
or delicately tinted lacquers. 

The B.E.S.A. specification differs in some particulars from the 
B.G.8. The limits of specific gravity are given as 0-769 and 0-801 
at 15°, and a distillation test is included providing that 95° must 
distil between 55° and 60° at 760 mm. pressure. ‘Titration tests are 
included to ensure that the acidity and alkalinity do not exceed 
0-01% calculated as acetic acid and as caustic soda respectively. 
The time in the permanganate test is reduced to 10 minutes, and the 
amount of acetone in the sample is estimated by a titration method. 

Until shortly before the war, acetone was derived entirely from 
the distillation of wood, and the separation of pure acetone from the 
crude distillate, containing water, methyl alcohol, acetic acid, 
various other ketones and pyroligneous impurities, is technically 
difficult. Large-scale experiments on the fermentation of starch 
by a special ferment, yielding butyl alcohol and acetone, were begun 
in 1912, and the process was largely developed during the war. 
According to Tunison,® contamination difficulties in the fermentation 
process have only been successfully overcome during the last two 
years, and the increased production from this source is likely to 
reduce the price of acetone in future.’ 

Methyl Ethyl Ketone, CH,:CO-C,H;—This substance is the 
homologue of ordinary acetone, which is dimethyl ketone, and is 
derived from the products of the distillation of wood. It has usually 
a pale yellow colour, which should not be too pronounced. Its 


- 102 Cellulose Ester Varnishes 


specific gravity varies from 0-810 to 0:815 at 15°, and on distillation 
95% should distil between 70° and 81°. It should be miscible with 
an equal volume of benzene and with three volumes of water, and 
should be neutral and free from non-volatile impurity. The 
B.E.8.A. specification (2D. 1) includes a test for refractive index, 
solubility in a saturated salt solution, and content of ketone, 
determined by a titration method based on oxime formation with 
hydroxylamine hydrochloride. Methyl ethyl ketone always con- 
tains some acetone, which is estimated with it and calculated as 
methyl ethyl ketone. This explains why it is possible to fix the 
minimum content as high as 100%. Pure methyl ethyl ketone is 
not a solvent for cellulose acetate at ordinary temperatures. 

Methyl Acetone—Methyl acetone is a crude fraction of wood 
distillate containing acetone and methyl alcohol as its principal 
ingredients. It varies very considerably in properties, as the following 
results of distillation will show :— 


Range. No. 1. No. 2. No. 3. No. 4. No. 5. 

Up to 50° a few a few a few a few a few 

drops drops drops drops drops 

50—55 2% 2% 31% 2% 35% 
55—60 44 54 49 36 54 
60—65 49 34 15 6 9 
65—70 4 9 4 1] 1 
Above 70 — — — 44 — 


The colour is usually yellow or yellowish-green and the odour 
very rank. Sometimes both the colour and the odour improve 
slightly after an exposure to light for several days. 

Higher grades can be obtained, however, and the material has 
been standardised by the British Engineering Standards Association 
(No. D. 2). It should be colourless or not darker than a faint 
yellow; 95% should distil between 50° and 70°. (It will be noted 
that all but No. 4 of the samples described above comply with this 
distillation test.) The acidity or alkalinity to phenolphthalein must 
not exceed 0-02%, and it must be miscible with water in all propor- 
tions or with its own volume of carbon disulphide at 25°. 

This specification is of particular interest, as it makes use of a 
determination of solvent power number (q.v.). Two solutions are 
made up containing 6 grammes of cellulose acetate (D. 6), one with 
‘ pure dry acetone ”’ and the other with the sample of methyl acetone 
under examination. A measured volume of each solution is placed 
in a flask immersed in a thermostat at 25°, and alcohol (2D. 9) is run 


“a 





Ingredients of Cellulose Ester Varnishes 103 


into each from a burette, with constant shaking, until a faint perma- 
nent opalescence is obtained. It is evident that if acetone is the 
better solvent, it will bear a greater dilution with alcohol than the 
methyl acetone before the cellulose acetate begins to precipitate ; 
and that the solvent power is measured by the volume of alcohol 
added. If the acetone solution has taken a c.c. of alcohol and the 
methyl acetone solution b c.c. of alcohol, then 


Solvent power of sample = : x 100. 


The material is graded according to the result of this test. 


Grade I has solvent power not below 65, 
99 II 9 9 99 50, 
29 ITI 29 29 ” 30. 


Methyl acetone in Grade I must also contain at least 55°% of acetone, 
determined by Messinger’s method (Ber., 1888, 21, 3366). 

It will be observed that the solvent power test only determines 
the value of methyl acetone as a solvent for cellulose acetate, and this 
is roughly proportional to its content in acetone. Since the other 
main ingredient is methyl alcohol, which is a solvent of nitrocellulose, 
methyl acetone is a better solvent for cellulose nitrate than for 
cellulose acetate. It is not a very desirable material, however, on 
account of its variability and its usually unpleasant odour. 

Ethyl Alcohol, C,H;-OH.—Ethyl alcohol, usually termed simply 
alcohol, and sometimes “ spirit,” is a constituent of nearly all cellulose 
nitrate lacquers, since they are made chiefly from nitrocellulose 
which has been dehydrated with alcohol. As is well known, it is 
produced by the fermentation of dextrose, derived either from 
starchy materials or from molasses. Other methods of manufacture, 
such as by the addition of the elements of water to the gas ethylene, 
CH,:CH,, have been and are being investigated, but have not yet 
had any influence on the market. 

Alcohol for use in the manufacture of varnishes is denatured 
(rendered non-potable) by the addition of 5% of wood naphtha, and 
is bought free of duty as “ Industrial Methylated Spirits.’ The 
presence of this quantity of wood naphtha, which is an impure form 
of methyl alcohol, confers slight solvent power on the alcohol. 
Alcohol should be colourless, free from appreciable non-volatile 
residue, and miscible with distilled water in all proportions. The 
concentration of alcohol is usually ascertained from its density, 
which is conveniently determined by a delicate hydrometer. A 
special form of hydrometer, known as Sikes’s, is made for this 


104 Cellulose Ester Varnishes 


purpose, provided with a series of counterpoises by the use of which 
it is possible to measure with considerable accuracy the density 
of any concentration of alcohol from zero to 100%. If only strong 
alcohol is dealt with, this instrument is not necessary. The best 
specific gravity tables to consult in ordinary laboratory work are 
those of Thorpe. 

Industrial methylated spirit should have a proof strength of not 
less than 66 O.P., which denotes a liquid having a specific gravity 
of 0:8171 at 15-6°/15-6°, and containing 91:99, of ethyl alcohol 
(including the methyl alcohol used as denaturant) by weight, and 
94-7°%, by volume. A higher specific gravity shows the presence of 
an excess of water, and such a spirit should not be accepted. 
Occasionally a somewhat stronger spirit with a lower specific gravity 
may be encountered, and is, of course, unobjectionable. 

A distillation test should show a fraction of not less than 95% 
distilling between 76° and 79° at 760 mm. pressure, and the alcohol 
should be neutral in reaction. The B.E.S.A. specification (2D. 9) 
gives limits for acidity to phenolphthalein and alkalinity to methyl 
red, and states that the spirit should have an agreeable odour, a 
stipulation with which the Government Chemist would probably 
not agree. 

Methyl Alcohol, CH;,-OH.—Methyl alcohol is a useful low-boiling 
solvent of the lower nitrates of cellulose. It is produced with 
other substances in the dry distillation of wood, and whether its 
solvent power is due to the presence of traces of acetone is a 
question that has never been authoritatively settled. At any 
rate the purest form occurring in commerce is a powerful solvent. 
This grade is seldom, if ever, used in varnishes, as it is dutiable. 
There is a lower grade usually sold as wood spirit, and the lowest 
grade of all is the so-called “ miscible wood naphtha,” selected 
for its nauseousness and used for the denaturing of ethyl alcohol 
(v. supra). 

Wood spirit cannot usually be used for transparent or delicately 
tinted lacquers, as it is seldom free from colour and often 
colours still more strongly on exposure to light. If it is required 
for a purpose where colour is of importance, this property should 
be tested with the samples submitted. It should leave no appreci- 
able residue on evaporation and on distillation should give 95% 
between 63° and 68°. The specific gravity 15°/15° should not 
exceed 0-810, and the spirit should be neutral. 

It should be noted that both industrial methylated spirit (ethyl 
alcohol) and wood spirit (methyl alcohol) invariably contain a small 


Ingredients of Cellulose Ester Varnishes 105 


percentage of water, so that neither is miscible with petroleum 
spirit without cloudiness. According to Tunison,® the use of 
anhydrous ethyl alcohol for the manufacture of lacquers is being 
developed in the United States, with the object of reducing as far 
as possible the original water-content of the lacquer. 

Hiher.—Ether, or diethyl ether, (C,H,),0, is made by the inter- 
action of sulphuric acid and alcohol; hence it is sometimes termed 
“sulphuric ether.” It is not an important commodity in the 
varnish industry, as it enters only into the manufacture of the 
ether—alcohol collodions, which represent a very small proportion 
of the total output. In the explosives industry it takes a much 
more important place. 

Ether is a colourless, mobile, volatile liquid, boiling at 34-5°, and 
is the most volatile solvent used in the industry. It is only partially 
miscible with water. The specific gravity of the commercial article 
varies from 0-72 to 0-73, according to the alcohol and water content. 
The specific gravity of pure ether is 0-7195 at 15°, and if the chief 
impurity is alcohol, a specific gravity of 0-730 corresponds with a 
material containing about 92% of ether, and a specific gravity 
of 0-720 with one containing 98—99% of ether. Anhydrous 
_ ether should show no cloudiness when mixed with an equal volume 
of carbon disulphide at 15°; the usual commercial article is miscible 
with alcohol, benzene and chloroform. Ether sometimes contains 
a small quantity of residue with an unpleasant odour, which is 
disadvantageous in collodion manufacture. This is most conveni- 
ently detected by allowing a few cubic centimetres to evaporate in 
an open dish. 

Although ether is so volatile, its vapour is so heavy, on account 
of its comparatively high molecular weight, that it may travel along 
the ground for a considerable distance in sufficient concentration to 
propagate a flame. Hence special care must be taken both in 
manufacturing and applying solutions in which ether is used. 
Running motors are the most likely source of sparks near the ground, 
and these should not be used in the same room as ether. 

Kithyl Acetate.—Ethy] acetate has hitherto been used much more 
in the United States than in this country, and Tunison ® refers to 
it as the most important nitrocellulose solvent. It is now made in 
this country from industrial methylated spirit under special Customs 
regulation, and the consumer must give a guarantee that it will be 
used for solvent purposes only, and not for such purposes as flavour- 
ing, where it would displace a dutiable article. The following is a 
typical specification :— 


106 | Cellulose Ester Varnishes 


It should be a clear, colourless liquid, with specific gravity at 


15° not less than 0-890 or more than 0-900. 90% should distil 
between 70° and 85°. (The boiling point of pure ethyl acetate is 
77°.) Its acidity calculated as acetic acid should not exceed 0-01%, 
and it should be miscible at 15° with double its volume of benzene. 


The ester content is determined by saponification with standard | 


alcoholic caustic potash in the usual way and should not be less 
than 85% calculated as ethyl acetate. 

Ethyl acetate has a higher specific gravity than ethyl alcohol 
(from which it is made) and a lower specific gravity than water. 
Hence a high figure for the specific gravity does not necessarily 
indicate a high ester content. It may indicate dissolved water and 
should be checked by the miscibility test. A low specific gravity 
usually indicates excess of unesterified ethyl alcohol. 

The B.E.S.A. specification (D. 19) is less stringent than that just 
given in regard to acidity, as it fixes the maximum at 0-1%. It is 
more stringent in regard to distillation (95% between 70° and 80°) 
and ester content (90%). 

A curious sample of American origin analysed in the writer’s 
laboratory a few years ago gave an ester content of 93:4%, was 
free from water and satisfactory in regard to acidity. The dis- 
tillation, however, gave only 71% up to 85°, 20% from 85—100°, 
and 7°% from 100—113° (total 98%). Apparently the higher boiling 
fraction consisted chiefly of a homologous ester. 

Butyl Acetate-——This material is displacing amyl acetate to a 
very considerable extent as the chief high-boiling solvent con- 
stituent of lacquers. This is largely due to its greater stability of 
price, since amyl acetate is generally to be preferred on account of 
its somewhat slower rate of evaporation and smaller affinity for 
water. It is made by the acetylation of butyl alcohol. 

Butyl acetate should be a clear, colourless liquid, free from 
appreciable non-volatile residue and having a specific gravity at 
15° between 0:875 and 0-880. On distillation 95% should boil 
between 110° and 130°. It should be miscible in all proportions 
with benzene, although the commercial article will sometimes turn 
cloudy with 4 volumes of petroleum spirit. The ester content 
should be at least 87-5% calculated as butyl acetate. 

The B.E.S.A. specification is 2D, 4. 

Amyl Acetate——It was explained in an earlier chapter ise the 
discovery by Stevens of the valuable properties of amyl acetate as a 
solvent for nitrocellulose laid the foundation of the lacquer industry. 
Unfortunately, ever since the rise of the cinema film industry, amyl 


Ingredients of Cellulose Ester Varnishes 107 


acetate, or rather the fusel oil from which it is made, has been a 
favourite commodity for the speculator. Its sole commercial 
source is as a by-product in the fermentation of sugar to alcohol 
and there is no competitive method of manufacture. The effect 
of the rapid fluctuations in price has been greatly to stimulate the 
manufacture of butyl alcohol and butyl acetate. 

Technical amyl acetate should be a clear, colourless, neutral 
liquid, free from appreciable residue on evaporation. The per- 
centage of ester calculated as amyl acetate should not be less than 
90, and 98% should distil between 110° and 145°. The specific 
gravity should not be less than 0-870 nor more than 0-875; and the 
ester should be miscible with benzene or toluene in all proportions. 
The B.E.S.A. specification is a little stricter in regard to distillation 
ranges, which is laid down as 95% between 125° and 145°. In effect 
this reduces the permissible amount of lower homologues, derived 
from the presence of propyl and butyl alcohols in the original fusel 
oil. 

Hthyl Lactate—According to Tunison,® the use of ethyl lactate 
as a nitrocellulose solvent is increasing rapidly. 

Technical ethyl lactate is a colourless liquid boiling from 145° 
to 155°, and its specific gravity is 1-037 at 20°. It is therefore 
somewhat heavier than amyl acetate, and evaporates more slowly. 
It will bear heavier dilution with water than most solvents without 
losing its solvent power. According to Gardner,’ solutions of 
nitrocellulose in ethyl lactate have a rather higher viscosity than 
those of equal concentration in butyl acetate, and dry more slowly. 
They will, however, bear considerably more dilution with non- 
solvents such as benzene. 

Acetone Oils.—The materials appearing in commerce under the 
name “acetone oil” are fractions, varying in composition, of 
higher boiling point than acetone, obtained in the wood distillation 
industry after the removal of the acetone. They consist of higher 
ketones, and a good sample may contain a considerable quantity of 
low-boiling ketones such as methyl ethyl ketone and methyl propyl 
ketone. Such solvents are known as light acetone oils and are 
useful in the industry. Heavy acetone oils are of much smaller 
value ; not only do they dry very slowly, but the smell is sometimes 
difficult to tolerate. 

No standard specification can be suggested, owing to the varia- 
bility of the material, and acetone oil cannot be commended as a 
solvent for the same reason. All that can be said is that the value 
of the oil is governed chiefly by its content in low-boiling material, its — 


108 Cellulose Ester Varnishes 


colour, and its odour, provided that the usual tests for neutrality 
and residue are satisfied. The acetone oils were first used in nitro- 
cellulose lacquers by Crane. 

Diacetone alcohol was patented by Doerflinger © in 1911 as a 
solvent for cellulose acetate and nitrate. It is prepared by the 
condensation of two molecules of acetone by sodium hydroxide, and 
has the formula 

(Hs 
HO—C—CH,—CO—CH,, 
CH, 


and the boiling point is 164°. A sample examined in the writer’s 
laboratory gave the following figures :— 


Specific gravity at 15° 0-9037. 
Completely miscible with water. 


Distillation— 
Up to 65° : MP eS 
65— 80° ; ; : - ee 
86-1409 
140-—164%° 
164-1709 ee 


The sample contained a considerable quantity of acetone. A 
solution of nitrocellulose in the sample gave a clear film on glass, 
which dried very slowly owing to the low vapour pressure of the 
bulk of the solvent. It is also a solvent of cellulose acetate, but has 
been used to a larger extent in the United States than in Europe. 

Tetrachloroethane, C,H,Cl,.—Tetrachloroethane is made by the 
combination of acetylene and chlorine. Acetylene tetrachloride 
and ‘‘ westron’’..are synonymous. It has the somewhat high 
boiling point of 146°, and is a good solvent for cellulose acetate when 
mixed with alcohol. Its specific gravity is 1:6 at 15°. In the 
presence of moisture it is liable to develop traces of hydrochloric acid, 
and it should be tested by shaking it with water, acidifying the 
aqueous layer with nitric acid and adding silver nitrate. It is not 
a solvent for cellulose nitrate under any conditions, and is not an 
ingredient of the official cellulose acetate aeroplane dopes. The 
odour is characteristic and easily recognisable. Whenever it is 
used, special attention must be paid to the ventilating arrangements, 
as the continued inhalation of the vapour is injurious and a number 
of cases of toxic jaundice among workers in the Benin por doping 
sheds in 1915—16 were traced to its use.11 


Ingredients of Cellulose Ester Varnishes 109 


Benzene, C,H,.—This is a coal tar distillate, to be carefully dis- 
tinguished from benzine, a paraffin distillate. To prevent the con- 
fusion arising from the identity of pronunciation, the former is usually 
known by its commercial name benzol, and the latter is frequently 
called petroleum spirit. Benzene approximates to a pure aromatic 
hydrocarbon, C,H, while benzine is a mixture of several homologous 
saturated hydrocarbons. Benzene forms with alcohol a mixture 
having weak solvent power for the lower nitrates of cellulose. 
Benzine is a non-solvent under all conditions. 

There is no need to stipulate for a high degree of chemical purity 
in benzol required for varnish manufacture, since its next homologue, 
toluene, C,H,-CH,, is also a useful diluent which forms a similar 
solvent mixture when mixed with alcohol. If required for clear or 
delicately coloured lacquers, it should itself be practically colourless. 
It should be free from acidity, and should leave no appreciable 
residue on evaporation. Its specific gravity should be not less than 
0-883 or more than 0-886 at 15°. When shaken with pure concen- 
trated sulphuric acid it should not give more than a pale yellow 
coloration. The B.E.S.A. specification (2D. 10) limits the amount 
of carbon disulphide, and includes a test to prove the absence of 
paraffin hydrocarbons. Carbon disulphide is an unpleasant impurity, 
as it not only affects the workers using the lacquers, but may also 
tarnish metal work by forming metallic sulphides. In fact, a 
rough but useful test is to note the time for the development of a 
stain on a clean silver coin. Paraffin hydrocarbons may be present 
as adulterants, since they are cheaper than benzol, but the sulphona- 
tion test described is not necessary with every consignment from a 
reputable supplier. 

Toluene, C,H,-CH,.—Toluene comes next to benzene in the series 
of aromatic hydrocarbons derived from coal tar, and the specifica- 
tion runs on similar lines. It can be obtained commercially in a 
high state of purity, perhaps higher than is necessary in the majority 
of varnishes. The distillation of such toluene should give at least 
95% boiling between 110° and 112°. The specific gravity should be 
not less than 0-867, not more than 0:869 at 15°. The colour developed 
on shaking with concentrated sulphuric acid should not be darker 
than straw-yellow. Toluene was prepared on a large scale during 
the war by the fractional distillation of Asiatic petroleum. Toluene 
of this origin should be tested by the sulphonation test (B.E.S.A. 
2D. 10) for the presence of saturated hydrocarbons.'* This test 
is unnecessary for toluene derived from coal tar. 

Commercial distillates can be obtained consisting chiefly of a 


110 ~ Cellulose Ester Varnishes 


mixture of benzene and toluene, and these can frequently be used in — 
varnishes. The specification in this instance should be based on a 
distillation test and specific gravity test, designed to limit undue 
variation in the proportions of the ingredients and hence in the 
rate of evaporation. 

Butyl Alcohol.—Buty] alcohol is a useful modern addition to the 
ingredients of cellulose nitrate varnishes. Although not a solvent 
itself it forms solvent mixtures with organic esters and with aromatic 
hydrocarbons, and on account of its low vapour pressure it checks 
the rate of evaporation, thereby preventing the deposition of water, 
and producing a smoother and tougher film. It should be a clear 
colourless neutral liquid, having a specific gravity at 15° not less 
than 0-810 or more than 0-820. 95% should distil between 95° 
and 120°. The B.E.S.A. specification (D. 17) contains a deter- 
mination of the percentage of butyl alcohol present, based on 
acetylation with pure acetic anhydride in a pyridine medium and 
titration of the acetic acid produced :— 


(CH,-CO),:0 +- C,H,-OH — C,H,:O-CO-CH, ae CH,-COOH 


This method is due to Verley and Bélsing,!* whose original paper 
should be consulted. It is, of course, not specific for butyl alcohol. 
Spiers 14 describes a method of determination of butyl alcohol by 
fractional distillation. 

Amyl Alcohol.—Crude amy] alcohol from the distilleries is known 
as fusel oil, and when purified by rectification is called amyl alcohol. 
These substances have similar properties to butyl alcohol, which is 
largely displacing them. 

Both should be neutral colourless liquids. The specific gravity 
of fusel oil ranges from 0-813 to 0-820, and of amyl alcohol from 
0-815 to 0-819. Fusel oil should distil between 125° and 132°, 
amyl alcohol between 128° and 132°. They should mix with an 
equal volume of toluene without cloudiness. 

Triacetin is the triglyceryl ester of acetic acid, 

3 CH,"0:CO:CH, 
CH:0-CO-CH,; , 
CH,°0-CO-CH, 


and is used as a softening agent both in cellulose acetate and in 
cellulose nitrate dopes, but chiefly in the former. The B.H.S.A. 
specification is 2D. 11. 

Triacetin should be a clear, colourless liquid having a specific 
gravity at 15° not less than 1-16 nor more than 1:17. It should be 


Ingredients of Cellulose Ester Varnishes 111 


miscible with twice its volume of benzene at 0° without showing 
cloudiness (freedom from glycerin and water). The acidity should 
be less than 0-15°% calculated as acetic acid. The makers will 
guarantee a maximum of 0-:1%. The percentage of ester is deter- 
mined by saponification with normal aqueous sodium hydroxide, 
and should be not less than 97% calculated as triactin. 

Benzyl alcohol, C,H;-CH,OH, is used as a high-boiling solvent 
and softener in cellulose acetate dopes, and occasionally in nitro- 
cellulose solutions. It is made by the saponification of benzyl 
chloride, C,H;-CH,Cl, and it must therefore be tested for free 
hydrochloric acid and for combined chlorine. The B.E.S.A. speci- 
cation is 2D. 7. 

Benzyl alcohol should be a clear, colourless liquid with a specific 
gravity at 15° not less than 1-050 and not more than 1-055. On 
distillation, 95°% should be collected between 200° and 210°. Rather 
a higher tolerance than usual is allowed for non-volatile residue, 
namely 0-1 gramme (maximum) from 10 c.c. Free acid, calculated 
as benzoic acid, must not exceed 0-1%. It must give up no soluble 
chloride to distilled water on shaking. The chlorine content is 
determined by boiling under reflux with alcoholic potash and estimat- 
ing the soluble chloride formed; this method assumes that the 
chlorine is present in a form removable by hydrolysis (e.g., as benzyl 
chloride). Benzyl alcohol is determined by acetylation with acetic 
anhydride in pyridine solution (v. butyl alcohol). 

Triphenyl Phosphate.—This material is used both as a softener 
and as a flame-resister in cellulose acetate dopes. It has rather an 
interesting history. It was patented as far back as 1901 by Zthl? 
as a substitute for camphor in ordinary celluloid, but was never used 
on the large scale. Shortly before the war it was adopted as a 
constituent of acetate dopes, and was manufactured in very large 
quantities for that purpose. After the war, attempts were made to 
work off stocks by using it as a camphor substitute in ordinary 
celluloid in accordance with Ziihl’s original patent, but it has several 
drawbacks, and its use for this purpose is diminishing. There is 
some conflict of evidence as to its solvent properties. For some 
varieties of nitrocellulose, a solution of triphenyl phosphate in 
ethyl alcohol has practically no solvent power whatever. 

The B.E.S.A. specification is 8D. 12. Triphenyl phosphate 
should be a white, dry powder or crystalline substance, which 
should not darken on exposure to sunlight. It should be com- 
pletely soluble in amyl acetate, butyl acetate, acetone, benzene and 
alcohol. The melting point should lie between 45° and 48°. The 


ita Cellulose Ester Varnishes 


acidity should be less than 0-01%, calculated as the equivalent of 
caustic soda. It should give a clear solution in chloroform (absence 
of water) and should be free from chlorides, sulphates and inorganic 
phosphates. The ash should be less than 0-05%. 

Castor oil is a valuable softening agent for nitrocellulose coatings, 
since its solubility in alcohol facilitates its incorporation in solutions, 
and when of good quality it has only a faint yellow colour. The 
best oil is that known as “ cold drawn.” 

The B.E.S.A. specification is 2D.5. The specific gravity should 
be not less than 0-963 nor more than 0-966 at 15°. The free acid 
determined by sodium hydroxide titration with phenolphthalein 
as indicator, should not exceed 0-75%, calculated as oleic acid. The 
iodine value should lie between 84 and 89. The solubility test in 
alcohol described in 2D. 5 is useful, but it is not clear why it is 
entitled ‘‘ Critical Solution Temperature in Alcohol,” since no 
critical temperature is determined. 

According to this test, 1 volume of the material wroule be 
completely soluble in 5 volumes of 59-6 O.P. alcohol at 20° and the 
solution should remain clear when cooled to 0°. (59:6 O.P. alcohol 
has the required specific gravity of 0-8303 at 60° F.) The refractive 
index at 20° should lie between 1-4772 and 1-4791. 

(Note.—The miscibility test described above appears to be based 
on that of Finkener. Archbutt and Deeley 1* recommend a variation 
of this test which has been of considerable service in the writer’s 
laboratory :—‘“* Measure exactly 10 c.c. of castor oil in a graduated 
stoppered test cylinder, add 50 c.c. of alcohol of specific gravity 
0-834 (= 89-9% by vol. = 57-6 O.P.) and well mix. If as little as 
5% of foreign oil be present the liquid will remain strongly turbid 
even on warming to 20°.’’) 

Castor oil is an expensive material, and unless it is bought from 
firms of the highest standing, adulteration is not uncommon. In 
doubtful instances it will be found useful to consult Lewkowitsch.*’ 

Resins.—The chemistry of the resins is unusually complicated, 
and for fuller details than can be given here, the reader is referred 
to the literature mentioned in the bibliography. 

Resins are incorporated with nitrocellulose varnishes for the 
purpose of increasing the body or solid content, and producing a 
harder, more elastic and more adhesive coating. The presence of 
the nitrocellulose renders the film tougher and more resistant than 
one derived from resins alone. The resins chiefly used in the 
industry are shellac, mastic, copal, sandarac, and dammar. 

Shellac—Shellac resin is the secretion of the lac insect, 
indigenous to India, and the production of shellac is an important 


‘\ 


Ingredients of Cellulose Ester Varnishes 113 


native industry in that country. It is a valuable resin on account 
of its hardness, adhesiveness and elasticity. The standard grade 


is that sold as <n, which usually contains 3% of added rosin. 


Garnet lac, which has a deeper colour, may have 10% of rosin, while 
the form known as button lac, consisting of small plates or discs of 
the resin, varies in composition from pure shellac to a product which 
is about half rosin. The addition of rosin (a cheap rosin which forms 
the residue from turpentine distillation) was at one time thought to 
assist the manufacture, and although it is no longer necessary, the 
existence of the adulteration is recognised in the industry. A 
bleached shellac is made by the action of alkaline hypochlorite on 


shellac. It is much paler in colour than <a> shellac, but more 


friable and not quite so soluble in methylated spirit. 

Shellac contains about 4% wax (insoluble in alcohol), the re- 
mainder being the resin. Garnet lac is free from wax and therefore 
gives a clear solution in alcohol, but against this advantage must be 
placed the higher percentage of adulterant. 

The pure resin is soluble in ethyl alcohol, methyl alcohol and 
amyl alcohol; partly soluble in ether, ethyl acetate and acetone; 
only slightly soluble in benzene, toluene and petroleum spirit. 

Mastic_—Mastic resin (commonly called gum mastic) is an 
exudation from a tree growing abundantly on the shores of the 
Mediterranean. It is a hard resin, but softens below 100°, and is 
pale yellow or greenish in colour. It is completely soluble in amyl 
alcohol, benzene and ether and partly soluble in most of the other 
usual ingredients of nitrocellulose varnishes, but is insoluble in 
petroleum spirit. It is, however, difficult to blend with solutions 
of other gums or with nitrocellulose on account of a tendency to 
precipitate, and considerable experience is necessary to obtain 
satisfactory results. It is sometimes adulterated with rosin or 
sandarac resin, as it is an expensive material. | 

Copal.—Copal is the name given to a large variety of tropical 
resinous. products, the best and hardest being of fossil origin. It 
occurs in East and West Africa, the East Indies, New Zealand and 
parts of South America. The colour varies from red to yellow. 
Zanzibar (East African) copal is usually considered the best, and 
New Zealand (kauri) is also very good. The best and hardest 
varieties, however, are not used in nitrocellulose varnishes, on 
account of their poor solubility. Softer copals are more soluble. 
For example, Zanzibar copal may contain 80—90% insoluble in 
geno, while the softer Manila copals are soluble to the extent of 


114 Cellulose Ester Varnishes 


about 95% in alcohol, and also dissolve in amy! alcohol and in amyl 
acetate. 

Sandarac.—Sandarac is the resin derived from a tree flourishing 
in north-west Africa, and another variety comes from Australia. 
It is a yellow, somewhat friable resin, occurring in commerce in 
small pieces or transparent drops of a yellow colour. It is soluble 
in ether, ethyl alcohol, acetone and amy] alcohol, but only partially 
soluble in benzene, toluene and petroleum spirit. It is moderately 
hard, and is used in the preparation of negative and label varnishes. 

Dammar.—Dammar is obtained from various trees growing in 
the Federated Malay States, Sumatra and the Dutch Hast Indies. 
It is a fairly hard resin, but it softens considerably below 100°. It 
is completely soluble in benzene and in oil of turpentine, and nearly 
so in ethyl alcohol. Itis partially soluble in ether and acetone. 

The resins show some properties resembling those of the cellulose 
esters, particularly in regard to their solubility relations. ‘They are 
frequently more soluble in a mixture of two liquids than in either 
alone, and this fact must be kept in mind in consulting the published 
information on their solubility data. 

Esselen notes that when lacquers are made up from solutions of 
cellulose esters and resins, the finished product is different according 
to whether the cellulose ester or the resin is added to the solvent. 
In some instances, if the resin is dissolved in the solvent first, the 
cellulose ester will not dissolve, while if the ester is added first, the 
resin will easily dissolve afterwards. He suggests that the cellulose 
ester and the solvent together form the dispersion medium, and the 
resin the disperse phase. 

The proportions in which the resins are used differ very consider- 
ably, according to the purpose for which the varnish is required. A 
typical formula given by Field, the originator of this kind of lacquer, 
was :— 


Amy] acetate : ; 4 . 650 gallons (U.S.) 
Oil of turpentine . ‘ ; . eae 
Methyl alcohol . ‘ ; a ee. 
Nitrocellulose : : ; . 87-5 Ib. 
Shellac . ; ; : f <a 


Worden remarks that the turpentine was omitted later, on account 
of its slow-drying properties. 

Two other formule may be quoted from Worden to illustrate the 
use of the resins. Both are for bronze paints having moderate 
resistance to heat, and therefore suitable for low- ‘hope 
radiators and hot water pipes. 


Ingredients of Cellulose Ester Varnishes 115 


Formula 1. Formula 2. 


TE ls s soca sacsnsennesceatesienasastasyes css 40—50 35—45 
STR oli acc ica yc ccd es sets ace aescoeceusacncs« 4—5 20—30 
ge ES a 35—50 15—25 
NS ks ccs reves as s4lsccedacanowsenge vanes 35—50 30—45 
IEE evo c cin icrvisccsgssksteicsagcvcceteeyece — 4t—6 
Nitrocellulose (not defined ).............cceceeceseseeeees 2-75 3:0 
Ee eu Gk ltkwn bys cnhuaesovecnccves¥enceceokanease 1:5 — 
eta ee cnc cco seSiuse vse te varscessapsscacets — 4-0 
Aluminium and bronze powder .............sceseeeeeee (added before 10-0 
use) 


The increased proportion of amyl alcohol in the second formula 
is probably necessary to keep the copal in solution, and the use of 
linseed oil is said to diminish the tendency of the paint to gelatinise, 
a drawback of bronze mediums to which reference will be made 
(p. 132). On the other hand, the addition of linseed oil diminishes 
the rate of drying considerably, and the film from such a solution 
will feel tacky long after one from a solution similar to formula 1 
will have set hard. 


REFERENCES. 

1H. A. Gardner and H. C. Parkes, Paint Manufacturers’ Assoc. U.S. 
Educational Bureau, Circular No. 218. 2 J. C. Wiesel, Chem. Age (New 
York), 1924, 32, 439. ° L. Clément and C. Riviére, Chimie et Ind., 1921, 
6, 283-295. * E. W. J. Mardles, J. Soc. Chem. Ind., 1923, 42, 1287. 
5 A. Marshall, J. Soc. Chem. Ind., 1904, 238, 645-648. ®B. R. Tunison, Chem. 
and Met Hng., 1925, 32, 93-94. 7 E. W. Blair, T. S. Wheeler, and J. Reilly, 
J. Soc. Chem. Ind., 1923, 42, 367 (‘‘ On the Fermentation of Maize Starch 
to n-Butyl Alcohol and Acetone’’). 8 Sir T. E. Thorpe, ‘‘ Aleoholometric 
Tables”? (Longmans, Green). °® H. A. Gardner, Paint Manufacturers’ Assoc. 
U.S., Cire. 225, Jan. 1925. 19W.F. Doerflinger, U.S.P. 1,003,438/1911. 141 (a) 
W. H. Willcox, Lancet, 1915, 188, 544; (6) W. H. Willcox, B. Spilsbury, 
and T. Legge, Trans. Med. Soc. London, 1915, 38, 129; (c) Annual Report 
of Chief Inspector of Factories, 1917 (Cd. 9108), p. 18 (Report on Doping in 
Aircraft Works, by W. H. Smith). 12 H. G. Evans, J. Soc. Chem. Ind., 
1919, 38, 402-405r (‘“‘On the Estimation of Saturated Hydrocarbons in 
Toluene ’’). 1% Verley and Bolsing, Ber., 1901, 31, 3354. 14 C. H. Spiers, 
J. Soc. Chem. Ind., 1924, 48, 251-2527. 15 EK. Zihl, E.P. 8072/1901. 
16 T,, Archbutt and R. M. Deeley, ‘‘ Lubrication and Lubricants ”’ (1912). 
17 J. Lewkowitsch, “‘Chemical Analysis of Oils, Fats, and Waxes” 
(Macmillan). 

| Additional References to Solvents. 

J. R. Lorenz, J. Amer. Leather Chem. Assoc., 1919, 14, 548-556. <A. D. 
Conley, J. Ind. Hng. Chem., 1915, 7, 882. 


Bibliography on the Resins. 


E. J. Parry, ‘‘ Gums and Resins”’ (Pitman). R. 8S. Morrell, “‘ Varnishes 
and their Components ’’ (Oxford Technical Publications). Allen’s ‘‘ Com- 
mercial Organic Analysis”? (Churchill), Vol. IV. G. J. Esselen, jun., 
‘Colloidal Behaviour” (edited by R. H. Bogue) (McGraw-Hill). E. C. 
Worden, “ Nitrocellulose Industry,”’ Vol. I, chap. x. T. H. Barry and R.S%. 
Morrell, “‘ Resins: Natural and Synthetic”’ (Benn). 


British Engineering Standards Association Specifications. 


Methyl Ethyl Ketone, 2D. 1. Butyl Acetate, 2D. 4. Castor Oil, 2D. 5. 
Benzyl Alcohol, 2D. 7. Nitrocellulose Syrup, 2D. 8. Alcohol, 2D. 9. 
Benzol, 2D. 10. Triphenyl Phosphate, 3D. 12. Acetone, 2D. 22. Cellulose 
Acetate, D. 50.—To be obtained from the Secretary of the Association, 
28 Victoria Street, Westminster, S.W. 1., price 2d. post free. 


CHAPTER VIII 
MANUFACTURE AND APPLICATION 


Manufacture of the Varnishes—Mixers—Measurement of Ingredients— 
Addition of Solvents to Esters—Incorporation of Pigments—Clari- 
fication and Filtration—Aeroplane Dopes—Effect on Strength of Fabric 
—Composition of Dopes—Doping Schemes—Method of Application— 
Discussion of Typical Dopes—Differences in Allied Practices —-Protec- 
tion from Sunlight—Metal Coatings—Application by Brushing, Dipping 
and Spraying—Transparent Lacquers—Enamels—Aluminium and 
Bronze Mediums—Motor-car Enamels—Imitation Leather—Leather 
Dressing—Patent Kid—Patent Leathers—Treatment of Motor-car 
Upholstery Leather—Coatings for Wood, Paper—Preservation of Antiques 
—NMiscellaneous Applications—Collodion. 


Manufacture of the Varnishes 


THE varnishes are made in any convenient type of mixing 
apparatus. Enclosed mixers provided with stirring blades are very 
efficient. Various types of revolving churns can be employed, or 
earthenware or lead-lined vessels with metal or wooden stirrers. 
Rapid stirring is neither necessary nor desirable, as the rate of 
dispersion of the esters is slow. Since in all instances some at least 
of the solvents are highly volatile, the mixer must be built as airtight 
as possible to avoid loss of solvent and change of composition during 
mixing. It is advantageous if the mixer can be easily cleaned, but 
it is preferable to use a separate mixer for solutions of each colour. 
If it is ever necessary to use a machine for an entirely different 
colour from that for which it has previously been used, it should 
be cleaned out by hand as completely as possible, and then washed 
out for several hours with a strong solvent mixture. 

The solid constituents of the varnish are weighed out. Cellulose 
acetate is usually weighed dry. Cellulose nitrate is generally weighed 
wet with alcohol from the dehydration process. The required 
weight must then be calculated from the laboratory determination 
of the amount of dry ester in the sample, and the quantity of alcohol 
in the ester must be deducted from the quantity indicated by the 
formula, when the liquid ingredients are added. 

The liquids are usually measured by volume, although, owing 
to their high coefficient of expansion, this method is not very 
accurate unless the temperature of the shop is moderately constant. 
There is a great deal to be said for measuring all the liquid ingredients 
by weight. Unfortunately, there is no uniformity of practice in 
the trades dealing in solvents. The tendency is for the cheaper 
liquids, alcohol, benzene, toluene, xylene and petroleum spirit, 
to be sold by the gallon, and more expensive liquids, such as acetone, 
ethyl, butyl and amyl acetates, by the ton. If they were all sold 
by weight, the varnish manufacturers would probably adopt the 

116 


Manufacture and Application 117 


same system and ensure, not only a more accurate and uniform 
manufacture of their goods, but also considerable simplification in 
their stock-taking and book-keeping. 

It is sometimes good practice not to add all of the solvent 
mixture to the cellulose ester at one time. No general rule can be 
laid down, but there is frequently a saving in time if the whole of 
the non-solvents (if any) and only a part of the true solvents are 
placed in the mixer at starting. The remainder of the true solvent 
is added at intervals determined by experience. There are two 
possible explanations of the saving of time which sometimes results 
from this procedure. In some, in fact in most instances, the ‘‘ non- 
solvents ” include ingredients which increase the solvent power of 
the true solvents. These ingredients diffuse rapidly into the 
ungelatinised ester, probably causing it to swell without dissolving, 
and this may assist the later penetration of the true solvent. 
There may also be a physical disintegration of the ester when the 
composition of the liquids is just approaching the point at which 
they become a solvent, and such disintegration will expose a large 
surface to the action of the solvent. This may be illustrated on a 
laboratory scale by shaking cellulose nitrate with aqueous acetone 
containing enough water to possess no solvent action. If the 
proportion of acetone is gradually increased, a stage is reached at 
which the cellulose nitrate disintegrates to a powder, and the con- 
tinued addition of acetone leads to rapid dispersion. On the other 
hand, when pure acetone is used as the solvent, it is difficult to 
prevent the formation of large clots, enveloped in gelatinised ester, 
and exposing so little surface to the action of the solvent that 
dispersion is greatly delayed. 

_ No general rule can be laid down as to the time required for the 
manufacture of cellulose ester solutions, since this varies according 
to the viscosity of the cellulose ester, the nature of the solvents, the 
order in which they are added, the final concentration and the 
temperature of the shop. Usually, however, batches begun on 
one day and kept in movement overnight are ready the next day. 
Pigmented varnishes (enamels) may be made directly from pig- 
mented celluloid, by dissolving the celluloid in an appropriate 
mixture of solvents. Celluloid, however, is made from a cellulose 
nitrate of high viscosity, and a solution made from it will yield a 
thinner coating for the same consumption of solvent than one made 
from an ester of low viscosity. Another property of varnishes 
made directly from celluloid which may be disadvantageous is their 
comparative softness arising from the presence of camphor. 


118 Cellulose Ester Varnishes 


When enamels have to be made from the nitrocellulose base, 
great care is needed to ensure the complete incorporation of the 
pigment. A mere stirring of the pigment into the finished solution 
is rarely successful; the particles of pigment tend to adhere together 
and ‘‘ ball up,” so that they do not disperse uniformly in the solu- 
tion. The film from such a solution, whatever the solvent, will 
be rough and gritty, and the pigment will rapidly settle out of the 
solution on standing. It must be remembered that to ensure that 
every particle of pigment is surrounded by an envelope of solution 
requires the performance of a considerable amount of work against 
surface tension, and this work cannot easily be done by stirring. 
To obtain a satisfactory result, the solution and pigment should 
be ground together in a ball mill, preferably with porcelain balls, 
or the ester should be gelatinised with part of the solvent and ground 
on steel rollers with the pigment. The latter process is expensive 
in solvent, since there is a considerable amount of evaporation 
during the grinding. 

Clarification of the Varnishes. 

Solutions of commercial nitrocellulose or cellulose acetate are 
never quite transparent, owing to, the presence of undissolved 
fibres, ‘clots or adventitious impurities. Jor many purposes, the 
presence of a small quantity of insoluble matter does not affect 
the usefulness of the varnish. In making enamels, for example, 
the process of grinding in the pigment comminutes the impurities 
also, and any kind of clarification is unnecessary. Sometimes, 
however, it is necessary to obtain solutions of high transparency, 
and some treatment is necessary. 

According to Kirkpatrick,! in the Parlin works of E. I. du Pont 
de Nemours and Company, filtration is carried out in frame filter- 
presses, through filter-paper backed with a heavy canvas filter- 
cloth. This process is only feasible when the solution to be filtered 
is of low viscosity, and even then it is advantageous to allow some 
of the insoluble material to settle by standing. A solution of 
high viscosity filters with extreme slowness through a press even 
when free from gelatinous impurities. The cellulose ester solutions, 
however, always contain impurities of a fibrous nature, frequently 
swollen by the action of the solvent mixture, and these felt together 
and form an increasing hindrance to the filtration. 

The impurities in cellulose ester solutions can be thrown out 
satisfactorily by treatment in a centrifugal machine, but unless 
special precautions are taken there is a heavy loss of solvent during 
the process on account of the rapid movement of air over the surface. 


Manufacture and Application 119 


The method which gives the most satisfactory results is 
undoubtedly settling under the influence of gravity. This is 
simply carried out by allowing the solution to stand in tall, narrow, 
cylindrical vessels, preferably of earthenware, until the impurities 
have fallen to the bottom. Where special transparency is required 
this process may take weeks or months. The disadvantages are 
(1) that losses by evaporation may be heavy unless strict pre- 
cautions are taken, (2) that a great deal of money is locked up 
while the settling is taking place. Clément and Riviére? state 
that solutions settled in this manner for eight months are optically 
pure. The writer cannot quite confirm this, but the path of a beam 
of light through such a solution is certainly very nearly invisible. 
On the other hand, solutions clarified by filtration, and especially 
by centrifugal treatment, give an unsatisfactory optical test, and 
will sometimes, on standing, deposit a flocculent gel which will take 
longer to settle out completely than the original impurities which 
have been removed. Apparently in the clarification of these 
solutions time is a factor which cannot be completely ignored. For 
general purposes, however, optical purity is not required in varnishes, 
and subject to the criticisms made above, filtration and centrifugal 
treatment are satisfactory. | 

Ayres ® mentions an instance of a nitrocellulose lacquer, which 
contained a cloud of finely-divided cellulose impurities, being 
filtered satisfactorily after the addition of a small percentage of 
tricalcium phosphate precipitated in alcohol. Other filtering agents, 
such as kieselguhr, fuller’s earth, bone black, and barium sulphate, ° 
were found useless. 


APPLICATIONS OF CELLULOSE ESTER VARNISHES. 


Aeroplane and Airship Dopes. 


The properties required in an aeroplane dope are that it shall 
increase the tensile strength of the fabric on which it is deposited, 
and cause a permanent increase in the tension. It should possess 
maximum resistance against water, oil and petrol, it should protect 
the fabric against the destructive effect of strong sunlight and 
atmospheric exposure, and it should be incombustible. 

It has already been pointed out in an earlier chapter that the 
need for fulfilling the last condition has led to the adoption of 
cellulose acetate rather than nitrate as the basis of aeroplane doping 
systems. 

Dopes for use on the fabrics of machines lighter than air should 


oe 


120 Cellulose Ester Varnishes 


possess the additional property of being impermeable to gases, and 
they should be poor conductors of heat. 

Clément and Riviére,? who have made a special study of the 
mechanical properties of doped fabrics, give the following figures 
to show the order of increase in strength brought about by doping. 
The figures refer to breaking strain calculated per linear metre of 
the fabric; the thickness is not stated :— 


Warp. Weit. 
Material. pe ee fe ee ee 
Before After Before After 
coating. coating. coating. coating. 
LOOT Feat on stag Seerees Oye 1,730 k. 1,860 k. 1,800 k. 2,660 k. 
COLOR hao < vait ae wae 940 1,080 1,010 1,180 
ELS Stn lia srasam sou anan 1,200 1,500 1,490 1,740 


A number of tables are given by Ramsbottom ‘ in the Technical 
Report of the Advisory Committee for Aeronautics for 1913-14, 
from which the following is taken :— 
















Fabric | Fabric 


Fabric , ; 
Tests on dope A. before with with 
doping. 5 coats | 7 coats 
dope. 
Weight, gmake PREF cicentae ant 138-5 312-5 
MOpe /1.? ye sicdvess — (174) 
Strength, = i to ees are 1275 1775 
Increase in strength due to 
GODS, 4 [icokstaes eek sacs we 500 
Specific strength of fabric... 9-2 5-7 


Specific strength due to 
Saieedehinustaneesemnves — ’ ‘ ne 2:87 


These tests are made in the direction of the warp only, as the fabric 
is more uniform in that direction. The results given on the last 
line for the specific strength due to the dope were calculated to 
show what is the increase in strength for a given added weight of 
dope. The values represent :— 


Increase in strength (kilos/m.) 
Weight of dope (grammes/m?.) 





They show that the influence of successive coatings of dope becomes 
rapidly less, so that a compromise is adopted between the standard 
of strength required in the coated fabric, and the weight of the 
fabric. 


Manufacture and Application 121 


According to Drinker,’ the fabrics used by the Allies for 
aeroplanes during the war were :— 


Great Britain . : ; . Irish linen, cotton. 
United States . ; . Mercerised cotton. 
Italy : ‘ ; ; oa, SEL 

- France. : ; : . Silk and cotton. 


Composition of Aeroplane Dopes. 


The French air service used acetate dopes on all their aeroplanes. 
The British used a nitrocellulose dope on aeroplanes used for 
training, and on the war machines an acetate dope with a final 
coating of nitrocellulose varnish to protect the under-coats from 
the action of light. 

If a dope is made up from cellulose acetate dissolved only in 
volatile solvents, it dries with considerable contraction to a hard 
film, which easily cracks. It is possible to introduce high-boiling 
softening agents into the solution which counteract this hardness, 
and produce a flexible film, at the expense of the contractile power. 
Since the fabric is stretched tightly on the frames before it is doped, 
only a moderate degree of contraction is necessary in the dope. It 
is therefore possible to incorporate a certain amount of a softener 
and thereby increase the resistance of the film to sudden shock. 
Of the softeners used the most important are triphenyl phosphate, 
tricresyl phosphate, phenol, acetanilide, methyl acetanilide, triacetin, 
benzyl alcohol. 

A large number of softeners were tried during the war, but 
owing to their high price the use of many of these has been dis- 
continued. 


Non-inflammability. 


The term non-inflammable is a somewhat vague word applied 
to those substances which do not burn freely. Cellulose acetate is 
a non-inflammable substance, so called because if in the massive 
form it is ignited, the flame usually expires as soon as the source 
of heat is removed. It must be remembered that the property of 
non-inflammability depends to some extent on the physical state 
of the material. Cotton wool, for instance, is an inflammable 
material; but a highly compressed bale of cotton or cotton paper 
in the form of a book would be difficult to ignite. é 

The term incombustible is reserved for substances having a 
higher degree of non-inflammability. Even this term is perhaps 


122 Cellulose Ester Varnishes 


only relative, but in common language it is applied to those sub- 


stances which exhibit no fire risk whatever. 

Although cellulose acetate is non-inflammable it is not incom- 
bustible, and when applied to a fabric in conjunction with an organic 
‘“‘ softener,” the fire risk may become sufficiently great to become 
a danger on aeroplane fabrics. It is usual, therefore, to incorporate 
some substance which will reduce the inflammability of the film to 
a state approaching incombustibility. Triphenyl phosphate is the 
material most commonly used for this purpose. It combines the 
functions of softening agent and reducer of inflammability. Although 
it contains a considerable percentage of carbon, the phosphate 
residue is an efficient check on combustion. 


Application of Dopes. 


British practice in the application of cellulose acetate and 
nitrate dopes is described in the following specifications issued by 
the British Engineering Standards Association.* 
2p. 101, Feb. 1923, “‘ Properties of Aeroplane Doping Scheme.” 
2p. 104, Dec. 1922, * Air Ministry Doping Schemes.” 
4p. 100, Feb. 1923, “‘ Air Ministry Cellulose Acetate Dopes.” 

D. 23, July 1918, “‘ Nitrocellulose Syrup for the Manufacture of 
Dope.” 

D. 102, June 1918, “‘ Nitrocellulose Dope.” 

2p. 103, March 1922, “ Air Ministry Nitro Dope Coverings and 
Identification Colours.” 

Dp. 105, Feb. 1923, “ Pigmented Nitrocellulose Dopes for Use on 
Aeroplanes.”’ 


These valuable publications should be consulted by anyone interested 
in the application of cellulose ester varnishes, whether in reference 
to aeroplane protection or not, as many useful hints are given and 
tests described. Only a brief summary with comments can be 
given here. 

2p 104 describes the purpose of the doping schemes, namely, 
the production of a taut, smooth, air-tight surface. Special coatings 
are required for protection of the doped fabric from light, for 
increasing the reflection of heat and light rays, and for better 
protection from observation during night flying. Directions are 
given for the application of the dope, which follow closely the 
usual practice in the industry. The shop should be dry and at a 


* To be obtained from the Secretary of the Association, 28 Victoria Street, 
Westminster, S.W. 1, price 2d., post free. 


a 


Manufacture and Application 123 


fairly constant temperature of 65-70° F. Humidity, or the preva- 
lence of cold draughts, causes ‘‘ blooming ”’ of the films through the 
deposition of moisture. The standard of ventilation is a high one, 
namely, that the air should be completely changed thirty times 
per hour. This standard was laid down after the exhaustive inves- 
tigation into the subject of ventilation carried out as the result of 
the mishaps with tetrachlorethane in 1915-16 (v. supra). 

The dope is applied with a flat brush of stiff bristle, and the first 
coat must be well pressed into the fabric. The fabric must not 
be damp or cold. It must be free from weaving defects, and should 
be lightly and evenly stretched on the frames. It has been already 
pointed out from Ramsbottom’s figures that the first coat gives 
the greatest proportionate increase in strength. This is doubtless 
due to the penetration of the dope into the fabric in such a way 
as to surround the fibres. A minimum of one hour is allowed before 
the next coat is applied. Usually in the industry a longer time 
than this is given, particularly when the solution is brushed on, 
as there is a great tendency for the second coat to disturb the first. 
Brush-work with cellulose ester varnishes is entirely different from 
brush-work with turpentine-linseed oil paints. The rate of evapor- 
ation is very rapid, and there must be no attempt at smoothing-out. 
The planes are doped preferably in a horizontal position, which 
favours an even application, and the brush should move in the 
direction of the strands of the fabric. 

The allowance of dope per 100 square yards of fabric (not 
including waste) is from 14 to 18 gallons, for four coats, which 
corresponds with rather more than 2 oz. per square yard when the 
volatile solvent has evaporated. 

2p. 101 defines four classes of doping schemes, the requirements 
of a dope being that it must tauten the fabric, protect it from 
injurious light rays and from moisture. Different schemes are 
possible according to whether those requirements are met by one 
dope or different combinations of two dopes. This specification 
describes useful tests for determining tautening properties, adhesion, 
elasticity, brittleness, weight, inflammability and rate of burning. 

4p. 100 gives the formule of four different types of cellulose 
acetate dope, namely : 


(1) A clear dope for use in temperate climates (A.D.) 

(2) 9 ” ” 39 ” tropical 99 (A.D.T.) 
(3) ,, pigmented ,, ,, temperate __,, (A.D.P.) 
Bere i oi ey ay)” * SeOpions io (DP) 


124 Cellulose Ester Varnishes 


It will be worth while to consider one of these specifications in 
some detail, as it will show how much standardisation is necessary 
in order to ensure practical uniformity in the dope. 

The formula for dope A.D. is as follows :— 


Cellulose acetate . : 5 ; oo TBE 
Acetone : , : . 60 gallons. 
Methyl ethyl Keiene, , « - Aes 
Alcohol . : : ‘ : : Si Pee 
Benzol . A ; : t ; Maes te 
Benzyl! alcohol ; : ; } j se ie ae 
Triphenyl phosphate ; ; , : 15 I, 


The literature of cellulose esters abounds with formule stated in a 
similar form to this, and most of them are almost worthless, because 
the properties of the ingredients are not defined. In this instance, 
however, every one of the ingredients has been standardised by 
previous specifications. Leaving out what one may call the obvious 
limitations on acidity, basicity and non-volatile residue on evapor- 
ation, these previous specifications fix the following properties on 
which the behaviour of the dope entirely depends :— 


(1) Cellulose Acetate. The required solubility of this material 
in a test solvent made up in the same proportions as in dope 
A.D. has been strictly defined (2D. 50), and, most important of 
all, the viscosity of this solution between narrow limits. 

(2) The five liquid solvents, acetone, methyl ethyl ketone, 
alcohol, benzol and benzyl alcohol, have all been specified in 
regard to allowable water content, specific gravity and range 
of boiling point. These determinations limit the amounts of 
impurities which might otherwise be present in commercial 
materials, and therefore ensure to a satisfactory degree that 
the viscosity of the dope when made up will fall between the 
same limits as were observed in the sample. 

(3) The specification for the triphenyl phosphate employed 
ensures uniformity in the hardness of the film produced and 
in the degree of combustibility. 


Without the rigid specification of all these ingredients, the 
final dope might vary so enormously in its properties that it would 
be entirely a matter of chance whether it would be suitable for the 
purpose in view. 

It will also be of interest to consider the same formula in relation 


Manufacture and Application 125 


to the boiling points of the volatile solvents, as these will give an 
approximate idea of the changes occurring during evaporation :— 





Ingredient. Quantity. Range of B. p. 
PRC rcerepestessescravcccsese 60 gall. 95% from 55° to 60° 
Methyl ethyl ketone ............ 49): 9575 =». 10° to-81° 
SM uansinsisens oransessece if.) 95% ., 76° to. 79° 
aida Ss vipndeas coeds st Maes 95% , 75° to 85° 
a ee Te 95%. ,, 200° to 210° 
Triphenyl phosphate ............ 15 Ib. M. p. 45° to 48° 


In the first place it must be noted that two of the ingredients, 
alcohol and benzol, are, when taken alone, non-solvents of cellulose 
acetate, even when hot. When mixed together, however, although 
they are still non-solvents in the cold, they form a solvent mixture 
on warming. Evidently therefore they are of more value when 
present together than either would be if present alone. In the 
next place, the most volatile constituent is evidently the acetone, 
and this will evaporate at a faster rate than the other constituents. 
It is, however, present in much larger quantity than the others, 
so that acetone is likely to be present in slight excess right up to 
the end of drying. Alcohol and benzol, which are chiefly of value 
when present together, and which have approximately equal rates 
of evaporation, are included in equal proportions, and their total 
volume, 28 gallons, is sufficiently small to ensure that they will 
not be left in excess when the true solvents have evaporated. The 
methyl ethyl ketone helps to diminish the rate of evaporation of 
the solvents, and in the presence of acetone is itself a strong solvent. 
It will be noted, however, that practically all of the volatile solvents 
boil below 85°, and that there is nothing intermediate between these 
and the benzyl alcohol, which will be left when the former have 
evaporated. The vapour pressure of benzyl alcohol at ordinary 
temperatures is extremely small so that its rate of evaporation is 
low, and it is left almost intact in the film with the non-volatile 
triphenyl phosphate. 

This is a comparatively simple formula to study in this way, 
owing to the large excess of strong solvents in it. When the pro- 
portion of non-solvents or latent solvents is higher, it becomes 
much more difficult to forecast what is likely to happen as the 
solvents evaporate. Skill in this exercise is of great assistance in 
the preparation of new formule. 

2p. 103 specifies various nitrocellulose dope coverings, applied 


126 Cellulose Ester Varnishes 


for protective coverings over acetate dopes. One of these may 
be examined in a similar way :— 


Transparent Covering V. 114 


260 Ib. of Nitrocellulose : : : .  62-4b. 
Nitrocellulose {Aleta : : : ; ~~ 2 
Syrup 2p.8 (Butyl acetate : ; : . 1 
Butyl or amyl acetate . : . 184 gallons. 
Alcohol ; : ; } : Ga 
Benzol ; ; ; ; . ee 
Castor oil . , ‘ / Jee 


It will be noted that this formula involves a previous specification 
2D. 8 in which the quantities are expressed by weight. The reason 
for this is that the nitrocellulose syrup is highly viscous and difficult 
to measure by volume. 

The same remarks apply to this formula as to the previous 
acetate dope formula. It would be valueless for its purpose if the 
solubility of the cellulose nitrate, and its viscosity in the specified 
solvents, were not fixed within narrow limits, and if the proportion 
of the solvents were not defined so as to exclude the presence of 
impurities which even in small quantities might have a marked 
effect on the properties of the dope. 

The boiling points of the ingredients may be tabulated in the 
same way :— 


Ingredient. Quantity. Range of B. p. 
Bnbyl acetate) ics cess s'exudeveeees 38) galls. 95% from 100° to 130° 
PARC ID alah s sles x oue yea elemieamaes af OD, as 76° to 79° 
BOT Sy ings ha Sh kcaikectoneee 994 ee 9695" 45 75° to 85° 
LIAR DONO |. Vous cas. tidavkee dee tawe gan 77. Ib, 


This is an even simpler mixture than the last. The non-solvents 
are again alcohol and benzol, but they form a solvent when mixed. 
Alcohol also assists the solvent power of butyl acetate. Benzol is 
the ingredient present in smallest amount, and there is no risk of 
it being present in excess at any stage of the evaporation. If amyl 
acetate is used instead of butyl acetate, the rate of drying will. 
be somewhat slower. 

Castor oil, being non-volatile, is entirely left in the nitrocellulose 
coating of which it will form more than half. 


Manufacture and Application 127 


Note on Bibliography. 


Interesting comparisons between the practice of various countries 
are given in the papers by Drinker 5 and by Clément and Riviere.? 
The Germans used lighter dope films than the Allies, the weight of 
dope being usually from 1 to 1:5 oz. per square yard, whereas 
British experience preferred a coat of 2 oz. The formation of white 
patches during doping, known as “ blushing ” in the United States 
and as “ blooming ”’ in this country, was due to the use of damp 
or draughty shops for the doping process. There is a very high 
percentage of low-boiling solvents in the standard aeroplane dope 
and very considerable cooling takes place as the solvents evaporate. 
Hence there is every probability of an excess of moisture depositing 
on the film and precipitating the cellulose acetate unless special 
precautions are taken to keep air dry. In this country, where 
doping processes and shops had to be hurriedly improvised, it was 
not possible to insist on the control of humidity in the doping 
sheds, but in the United States advantage was taken of the experi- 
ence of the Allies and such control was enforced. 

Although some experimental work was carried out on doping 
by means of a spray, or air-brush, the process was carried on by 
hand-labour throughout the war. 

In French practice, the first or “‘ scratch ” coat contained only 
from 3-5% of cellulose acetate, and was made up with a high 
proportion of low-boiling solvent in order to give a greater tautening 
effect on the fabric. The second and third coats contained from 
8-9°% of acetate, with 2° of mineral pigments or metallic powders. 
Hither eugenol or triacetin was used as the softening agent in these 
coats. The last coat was made with 8% acetate, and with a solvent 
having a high proportion of low-boiling solvent, as in the first coat. 
The French used chiefly methyl acetate as low-boiling solvent, and 
the British chiefly acetone, but the British Air Ministry endeavoured 
to minimise the consumption of products of wood distillation by 
the use of alcohol and benzene as diluents. 

The French dopes did not contain triphenyl phosphate, but 
employed a variety of other softeners such as eugenol, benzyl 
alcohol, triacetin and glyceryl benzoate. None of these had a 
fire-resisting action similar to that of triphenyl phosphate. The 
Italian used a small percentage of phenol as the softening agent. 
The United States service used in the first coat a small percentage 
of phenol or naphthalene, and also an ant-acid, either urea or 
dicyandiamide. Three additional coats were put on, which con- 


128 Cellulose Ester Varnishes 


tained 7°% of diacetone-alcohol to prevent “ blushing,” and small 
quantities of benzyl acetate or benzyl benzoate as softening agents. 

The destructive action of sunlight on aeroplane fabric was 
investigated by Aston ® (of isotope fame) and was found to be due 
chiefly to ultra-violet light in the region of the spectrum lying 
between 2950 and 4000 wu. These rays could be absorbed by 
certain pigmented coatings, and a special protective dope was 
developed known as PC 10. The pigment in this covering consists 
of a mixture of yellow ochre and carbon black, in a nitrocellulose 
and castor oil medium. 

The various pigments employed to produce the colours used on 
British aeroplanes for the purposes of identification and protection 
are detailed in B.E.S.A. 2p. 103. Dopes which gave a matt black 
surface were adopted in France, at the suggestion of Clément and 
Riviére,? for machines used for night-bombing, and were almost 
invisible in the beam of a searchlight. Later suggestions from the 
same source were ‘clair de lune,” blue-violet and deep red. 


Metal Coatings. 


The application of special enamels made from nitrocellulose of 
low viscosity to motor-car bodies, as has already been pointed out, 
is a recent development which has taken place only in the last two 
years. For many years, however, an industry has flourished in 
the protection and decoration of small metal articles with nitro- 
cellulose lacquers. The use of lacquers and enamels of higher 
viscosity results in thinner coatings for the same consumption of 
solvent, or, conversely, a large number of applications to produce 
a thick coating. As most of the articles so coated are intended 
only for indoor use, where they do not encounter severe weather 
conditions, a thin coating is generally adequate for the purpose in 
view. 

There are three general methods of application, (1) brushing, 
(2) dipping, (3) spraying. 

Brushing is the least satisfactory process. Nitrocellulose lacquers 
dry so rapidly that any attempt to smooth out brush marks generally 
defeats its purpose. If used for a second application, it invariably 
disturbs the first coat, and the lacquer rapidly becomes sticky 
enough to draw bristles from the brush. Nevertheless, it may 
sometimes be necessary to apply a lacquer or enamel to a surface 
too large to be dipped conveniently, and in a situation where a 
compressed air spray is not available. Under these circumstances, 


Manufacture and Application 129 


fair work may be done if the operator resolutely banishes from his 
mind all preconceived notions derived from the application of 
ordinary linseed oil paints or varnishes. The lacquer should be 
quickly applied to the surface without brushing out, keeping the 
brush well moistened. Whenever work is stopped, if only tem- 
porarily, the brush should be immersed in a solvent mixture to keep 
it from drying out. Lacquers intended for brushing require a 
higher percentage of high boiling solvent, so as to delay the rate 
of drying. 

Dipping is a very satisfactory method of coating small articles. 
The lacquer is placed in a receptacle of convenient size, and the 
article is completely immersed and withdrawn. It is allowed to 
drain for a few seconds and is then transferred to a rack to dry. 
The two chief precautions to be taken are (1) to immerse the article 
in such a way that no bubbles of air are drawn under the surface ; 
the procedure depends on the shape of the article and can usually 
be determined after one or two experiments; (2) to drain the 
article in such a way that no “‘ drip ”’ is formed at corners or edges; 
in some cases it may be advisable to apply a slow rotation mechanic- 
ally to the articles while drying. A recent patent taken out by 
B. D. Baker’ claims that drip may be avoided by hanging the 
articles in a vessel of the lacquer, and slowly drawing off the lacquer 
from the bottom of the vessel. 

Spraying is a comparatively modern development, and is carried 
out with apparatus similar to that used in painting. Various types 
of air sprays are on the market, such as the Aerograph and the Aero- 
style. The principle of the apparatus is that a fine jet of air under 
somewhat high pressure draws the lacquer from a reservoir by its 
injector action, and propels it in the form of a fine spray towards 
the article to be coated. In some instances, the reservoir feeding 
the spray is also under pressure. The conditions which may be 
varied are the pressure of the air, the viscosity and composition of 
the lacquer, the ratio of air to lacquer, the temperature of the 
workshop and the distance of the spraying apparatus from the 
work. Most of these are under the control of the operator, but the 
composition of the lacquer is the concern of the manufacturers. 
The latter can, however, assist the operator by supplying the lacquer 
in a slightly more concentrated form than is required for the spray, 
and supplying separately a ‘‘ thinnings”’ or diluent, consisting of 
solvent only, which may or may not have the same composition as 
the liquid solvent in the lacquer. A few trials will usually show 
what . the best proportion in which to mix the lacquer and the 


130 Cellulose Ester Varnishes 


thinnings, and great care should be taken to ensure that the two 
are thoroughly mixed. 3 

Flowing is really a variant of the dipping process, and is some- 
times convenient when large articles are to be covered. The lacquer 
is allowed to flow from an orifice over the surface of the article, the 
excess which drains off being returned to the containing vessel. 

Another process of metal covering is intermediate between the 
use of solutions and the use of solid celluloid coverings. The 
lacquer or enamel is made in such a high concentration as to be a 
plastic semi-solid. This is applied to the article sometimes by 
mechanical means, sometimes by hand with a palette knife, and 
after the coating has partly dried it is moulded to the desired shape 
in a specially designed press. 

Coatings for metals may be classified as follows :— 


(1) Transparent lacquers, either colourless or coloured. 

(2) Enamels, white, black or coloured. 

(3) Metal-powder coatings, such as bronze or aluminium 
paints. 


These usually contain resins in addition to the nitrocellulose. 
The resins add body to the solution, and promote adhesion. 


Transparent Lacquers. 


Gardner quotes as a typical solvent mixture :— 


Butyl acetate ' : : . Mn hy = 
Ethyl acetate : . «2, 
Benzene : : A ; ~ Oe 


to a gallon of which would be added 6 to 8 oz. of nitrocellulose 
(viscosity not specified). It will be noted that this formula contains 
no alcohol, and would have to be made up from stove-dried nitro- 
cellulose. As it stands, it contains the solvents butyl and ethyl 
acetate in a sufficient proportion to ensure that they will be in 
excess of the non-solvent benzene at all stages of the evaporation. 
If nitrocellulose dehydrated with alcohol were employed, as is 
usual, the 8 oz. of nitrocellulose would introduce about 4 oz. of 
alcohol, which would not alter the properties of the lacquer appre- 
ciably. In fact, half of the benzene in the above formula might 
be replaced by alcohol. For use as a protective coating, Gardner 
recommends the introduction of 5-7% of castor oil or treated tung 
oil, giving a more “elastic”’ film. The term elastic is here used 
in its popular sense of ‘‘ extensible.” The film is actually softer 


Manufacture and Application 181 


and more plastic, and less likely to strip away from the metal by 
shrinkage. 

Wiesel suggests the following as a typical lacquer for highly 
polished surfaces :— 


Nitrocellulose (4 sec.) . . 5% by weight 
Alcohol . ; ; ae, 

Benzol ; : ; - . 20 

Butyl acetate ; ; jf . 40 

Butyl alcohol ; ; : ik 

Ethyl acetate : : ; vee 

Castor oil . : ; cate tere 


Here the viscosity of the nitrocellulose is indicated by the expression 
“4 sec.” This expresses the result of an empirical test of viscosity 
carried out by timing the fall of a steel ball of fixed weight through 
a distance of 10 inches in a solution of fixed concentration in a given 
solvent mixture. Such a test is quite definite as long as the con- 
ditions are observed, but there is much to be said for expressing 
the results in recognised scientific units. 

In this formula alcohol is present to the extent of 20%, so that 
the solution could be readily made up from nitro-cotton dehydrated 
in the usual way with alcohol. The precipitating tendency of 
benzol is amply safeguarded by the presence of alcohol, with which 
it forms a solvent mixture, and by the combined solvent power of 
the ethyl and butyl acetates. The butyl alcohol helps to slow the 
rate of evaporation, and assists the solvent power of the acetate. 
Also, like the ethyl alcohol, it forms a weak solvent mixture with 
the benzol. The mutual influences on solubility of the ingredients 
in this mixture are thus somewhat complicated. 

If either of these formulz were required for a colourless lacquer, 
attention would have to be paid to the colour of all the ingredients 
and of the original nitrocellulose. Either could be transformed 
into a coloured lacquer by dissolving in the liquid solvent the 
appropriate spirit-soluble aniline dye, or mixture of dyes. These 
lacquers are largely used in the fancy metal-ware industry, both 
for protection from corrosion and for decoration. 


Enamels. 


Similar formule with the addition of the requisite pigments and 
resins give the enamels. The pigments should be ground up either 
in the castor oil or in the plastic mass formed when the nitrocellulose 
is gelatinised with part of the solvent. 


182 Cellulose Ester Varnishes 


Enamels of this kind are used as waterproof paints on domestic 
metal-work, and on small articles such as motor-car accessories, 
scientific instruments, and sanitary goods. When the best results 
are required, it is usual to apply two or three coats, and to buff or 
rub down with fine glass-paper between the coats. The best effect 
of all is produced by giving a final dip or spray with a transparent 
celluloid lacquer, which gives a beautiful gloss to the surface. 

A matt surface is obtained by increasing the quantity of pigment 
and/or varying the proportion of resins. This type of surface 
appears to be due to the pigment particles not being completely 
surrounded by an envelope of the nitrocellulose, as they are in a 
glossy enamel. It is a somewhat uncertain process in practice, 
as a matt enamel may under some conditions of temperature and 
humidity dry brighter or duller than usual. A good matt enamel 
should not give up colour when rubbed with the finger. 


Aluminium and Bronze Mediums. 


These invariably contain resins in addition to the nitrocellulose, 
and they are applied to metal as much for protection as for decor- 
ation. The metal powders must be very fine, so as not to clog the 
jet in spraying work. 

When bronze powders are worked into nitrocellulose solutions 
it is sometimes noticed that after a few hours or days a green colour 
develops and the nitrocellulose begins to separate out in large clots. 
This is usually attributed to instability of the nitrocellulose, but it 
sometimes occurs with nitrocelluloses of proved stability, and 
when the solvents are apparently in good order. It is a phenomenon 
which requires investigation. It is better not to stock these solutions 
made up ready for use, but to work in the metal powder immediately 
before the paint is required. These paints are frequently applied 
with a brush, and have been used to a larger extent on outside 
work than any other nitrocellulose solution previous to the new 
motor-car finishes. 


Motor-car Enamels. 


The use of nitrocellulose enamels for covering the bodies of” 
motor-cars has been developed chiefly in the United States, and is 
competing with the existing methods, which are classified by 
Mougey ?° as follows :— | 


(1) Methods employing oil and resin varnishes, applied in 
a similar way to coach-builders’ varnishes. ‘The applications. 


Manufacture and Application 133 


may consist of one priming coat, any number up to seven 
surfacing coats, from one to five colour coats and a glossy 
finishing coat. 

(2) The use of stoving enamels, which require heating after 
application to temperatures varying from 60° to 200°. These 
may be applied in three coats, a primer, a colour coat and a 
finish. Black enamels containing bitumen are the commonest 
of the stoving enamels. 


Both of these methods may give excellent results, but they 
have certain drawbacks. Varnishing in accordance with the first 
method is costly in time and labour. The coats take several hours 
to dry, and there is a considerable amount of rubbing down to be 
done if the best results are required. As many as twenty-eight days 
might be required for the complete operation. 

Stoving enamels give a hard coating with a high lustre. Both 
processes, however, yield a coating which may have poor resistance 
to atmospheric conditions. The varnish coatings may develop 
large cracks, and the stove enamels, though more durable, some- 
times become covered with a vast number of very small intersecting 
cracks. Both finishes are easily scratched, and a varnish finish is 
sensitive to petrol, oils and tar. 

_ By the use of nitrocellulose enamels it is claimed that a finish 
may be obtained which is equal to that of the black stoved enamel, 
but without the need for stoving. Practically any colour can be 
obtained, and the solution can be applied by an air-spray. Com- 
pared with the first method, the number of coats can be considerably 
reduced, since an enamel of high nitrocellulose content is used, 
giving a thick coating which is dry in a very few minutes, and which 
can be polished to any desired degree. 

According to Flaherty,1! nitrocellulose enamels were first. used 
on motor-cars in some of the re-finishing shops in California, but 
at that time the amount of nitrocellulose in the solution was so small 
that several coats had to be used to give sufficient thickness of film 
and depth of colour. The process was therefore very expensive 
and not adapted to general use. 

The later development in motor-car enamels has been brought 
about entirely by the discovery of a method of producing stable 
nitrocelluloses of much lower viscosity than was formerly thought 
possible, so that the manufacturer can obtain a much higher solid 
content without making the enamel too viscous to spray. 

The coatings obtained by the new process are said to be more 


134 Cellulose Ester Varnishes 


stable than those made from oil/resin paints and varnishes, and to 
possess better resistance to tar and petrol. They can be cleaned 
down with a wet mop without becoming scratched, and although 
the original gloss is not so good as that obtained by the best coach- 
builders’ varnish or stoved black enamel, it can be brought up by 
rubbing and improves with age. The extra resisting power of 
nitrocellulose coatings is chiefly due to the fact mentioned in the 
first chapter, that the polymerisation of a drying-oil takes place 
slowly after the application of the varnish, whereas the nitrocellulose 
enamels already contain the polymerised base in the form of the 
cellulose ester. 

The saving in time is remarkable. Kirkpatrick states that the 
time required to finish the body of one particular make of car has 
been reduced from 336 hours to 134. The number of operations 
has been reduced by one-third, and the number of bodies in process 
at one time has been reduced from 2,400 to 600. Condé 3° states 
that the same firm were able to reduce the area of floor space given 
up to painting operations by 30,000 sq. feet, which in itself repre- 
sented a large capital saving. . 

The method of manufacture of low viscosity nitrocellulose 
enamels follows the ordinary procedure so far as details have been 
published, but the precise way in which the viscosity is kept so low | 
has been kept secret. The solvents employed are said to be chosen 
from amyl, butyl and ethyl acetates, amyl and butyl alcohols, 
xylene, toluene, benzene, ethyl alcohol and acetone, while among 
the gums mentioned in this connexion are shellac, soft copals and 
dammar. 

The method of application of the enamels has been described 
by Flaherty.1!_ Any previous coating of varnish or paint must be 
removed, e¢.g., by use of the blow lamp, as the nitrocellulose solvents 
_are liable to form soft spots over places where the previous varnish 
coat is not completely oxidised. When thoroughly cleaned, the 
metal is coated with an ordinary oxide primer, which must be 
allowed to oxidise fully before the enamel is sprayed on. The 
nitrocellulose enamel is then applied. 

Respirators are worn by the workmen while they are spraying, 
and in British practice the car body stands in front of a concave 
metal shield with an extraction fan in the centre designed to with- 
draw the fumes. As a rule, a fairly high air pressure is used, some- 
times as much as 80 lb. per square inch, but custom varies consider- 
ably in this respect. 

The use of a glossy oil-resin varnish over the nitrocellulose 


Manufacture and Application 135 


enamel is not recommended, as if a high gloss is required it may be 
obtained by friction. 

Failure of the enamels in service may be due to any one of four 
causes: (1) Improper preparation and oxidation of the under 
coats. (2) Too heavy a coating of enamel. (3) Spraying the 
enamel too dry, either through using too high a pressure, or keeping 
at too great a distance from the work. (4) Allowing the enamel 
to settle out in the container, for want of stirring. 


Imitation Leather. 


The imitation leather industry originated in the process of 
Wilson and Story (1884), who coated cloth with a solution of nitro- 
cellulose in a mixture of amyl acetate and castor oil.!2 The later 
developments have been chiefly in the use of more economical solvent 
mixtures, and more suitable varieties of nitrocellulose. 

The fabric coated is usually cotton, commonly a canvas duck, 
which should be free from weaving defects giving an irregular 
surface, such as irregularity in the thickness of the thread. Light 
fabrics are usually given a dressing to weight them. Heavy fabrics 
are lightly fluffed so as to assist adhesion. 

The solutions are applied to the moving fabric through an 
adjustable slot set to deliver the correct amount. The solvents 
chiefly used are acetone oil and ethyl acetate, diluted with alcohol, 
benzene and petroleum spirit. Oil must be added to ensure plia- 
bility of the coating. Castor oil is still the best, on account of its 
light colour and non-drying properties. It is said that blown 
linseed oil, colza oil and cotton-seed oil are also used, but they are 
liable to turn acid. The pigments are ground in the oil before 
introducing them into the solution. Resins are not employed. 

The number of coatings may vary from three to thirty, according 
to the thickness of material desired, but it is likely that the advent 
of nitrocellulose of lower viscosity will reduce the number required 
for thick material. The first coatings usually contain only a small 
quantity of oil, as they are required primarily to secure adhesion. 
The intermediate layers, which give body, contain more oil, and the 
final coatings, which give the gloss, contain neither oil nor pigment. 
The cloth must be dried between the applications. 

Tucker 1 discusses the question of viscosity. If this is expressed 
by the time taken for a steel ball, 5/16 inch in diameter, to fall 
through a distance of 10 inches in a solution made up from 16 oz. 
of nitrocellulose to 1 (U.S.) gallon of solvent, 70% ethyl acetate and 
30% benzene, the viscosity of the nitrocellulose used for artificial 


186 Cellulose Ester Varnishes 


leather is usually a 20-second cotton. When this is made up into 
the correct formula, including the diluents, the viscosity is about 
40 seconds. 

Deschiens 15 has described the preparation of artificial leather 
from cellulose acetate, but the higher cost of this ester limits its 
employment for this purpose. It could only compete if there were 
special reasons for producing a material less inflammable than the 
nitrocellulose leather. 

The imitation of the grain of real leather is put on by passing 
the coated cloth through engraved rollers under pressure, and 
sometimes the cloth is then rubbed by hand with a pad soaked in 
solvent containing a slightly darker pigment than that used in the 
coating solution. This pigment collects in the channels of the grain 
and produces an imitation of old leather. 


Leather Dressing. 


Nitrocellulose solutions are used to impart a high gloss in the 
so-called patent or enamelled leathers. 


Patent Kid. 


The processes used in making this material are described by 
Crockett.1® Three coats of linseed oil varnish are usually applied, 
and the skins are stoved between the applications. A nitrocellulose 
solution is sometimes used instead of the first coat or “ daub,” and 
is said to be a 3% solution in amyl acetate. The viscosity is not 
specified, but it must not be too low, or the solution will penetrate 
the leather too much and harden it. The daub is applied by means 
of a brush on the grain (outer) side of the leather, which has been 
previously cleaned with petroleum spirit to remove grease. The 
other two coats are applied above the nitrocellulose coat. They 
consist of linseed oil, blue and black pigments, and driers, boiled 
for several hours at 500° F. 


Patent and Enamelled Leathers. 


These are now frequently prepared entirely with nitrocellulose 
solutions, the older linseed oil varnishes having been largely super- 
seded. The leather should be thoroughly cleansed from grease, 
and quite dry. Three coats are applied, the first by means of a 
stiff brush, with which the solution is worked well into the leather. 
The second coat is applied by means of a woollen sheep-skin swab, 
made by tacking to a wooden handle a piece of sheepskin tanned 
with the wool on. The third coat is applied in the same way as 


Manufacture and Application 137 


the second, and must be applied thinly and evenly. Between the 
applications the skins are dried at 65-100° F. 

According to Clément and Riviére,? the solvents employed in 
the first coat are :— 


Amyl acetate : . 1800 parts 
Petroleum spirit . - ume DOO 3, 
Castor oil 

or 900 _—,, 
Boiled linseed oil 


The second coat contains less oil and the third none, so that a high 
gloss is obtained. 

The colour is usually a soluble (aniline) black. The leather 
may be embossed under a hydraulic press in order to give it any 
desired configuration. - 


Motor-car Upholstery Leather. 


Herrmann and Radel !” describe the treatment of this material 
in an article very fully illustrated by reproductions of photographs. 
The split hides are doped with nitrocellulose varnish. In this process 
the skins are stretched out on tables, and the first coat is put on 
with the same type of brush as that used by bootblacks. The later 
coats are applied with brushes made of Chinese hog bristle, six or 
seven inches long, having a wooden peg or “ dutchman”’ in the 
middle so as to allow an extra quantity of material to be carried 
and to give a spread to the brush. It is stated that this spread is 
essential to produce a good finish. 

The leather is sometimes finished with nitrocellulose dope alone, 
sometimes entirely with a japanning finish of boiled linseed oil, 
turpentine, naphtha, driers and pigments, and sometimes with a 
combination finish consisting of undercoats of nitrocellulose and 
finishing coats of a linseed oil japan. 


Wood Coatings. 


The use of nitrocellulose varnishes for the protection and decor- 
ation of wood at first sight presents a great advantage over French 
polishing in the great saving in time involved. Up to the present, 
however, there has not been great development in this direction. 
This may be partly due, as Clément and Riviére suggest, to the 
conservatism of the skilled French polisher, which is not unnatural 
in view of the excellent work which he turns out. Probably the 


138 Cellulose Ester Varnishes 


chief obstacle, however, has been the large consumption of solvent 
in the process, due to the absorption of the solution by the wood, 
and the first problem is undoubtedly to find a cheap coating to act 
as filler and primer. This is made possible by the use of modern 
low viscosity varnishes, which deposit much more solid content for 
the same expenditure of solvent.1® It is sometimes advantageous 
to begin by rubbing in a pigment of appropriate colour damped with 
methylated spirit, e.g., for a light coloured wood such as sycamore, 
plaster of Paris, finely-divided silica, chalk, or starch. Spirit 
varnishes (resins) may also be used as first coats, but the final 
effect is not usually so good. 

Wood covered with a thick coating of white nitrocellulose paint 
is much used for sanitary purposes, and for bathroom, surgery and 
hospital shelves. This is given a succession of applications, allowing 
a few hours for drying between each, and with frequent rubbing 
down with glass paper to ensure smoothness. The final coats should 
be given with an enamel of the purest white obtainable, and some- 
times a transparent coating is applied last of all to give a high gloss. 
Wood so treated is proof against damp and can be washed down 
without taking harm. 

Picture frames and other decorative articles in wood are fre- 
quently coated with bronze or aluminium paints. Matt black 
varnishes are employed for the interior of cameras and other optical 
instruments. 


Paper Coatings. 


Paper is sometimes coated with nitrocellulose varnish either for 
protective purposes or for decoration. Cardboard price labels for 
use in retail shops are frequently coated with a transparent solution 
drying with a high gloss. Cheap fancy articles may be made by 
coating stout paper in much the same way as in the manufacture 
of imitation leather, using a solution containing castor oil as softener, 
and soluble colours of pigments. The paper may then be embossed 
with any desired pattern by passing it through engraved rollers. 

Photographic papers are sometimes enamelled by coating with 
a transparent nitrocellulose varnish. Posters and time tables may 
be similarly treated to protect them against the weather. 


Preservation of Antiques. 
Solutions of cellulose acetate and cellulose nitrate have been 
largely used by Lucas 1° for the preservation of antiques from the 
recently opened Egyptian tombs. For this purpose he employed 


Manufacture and Application 139 


either ordinary celluloid dissolved in acetone, amyl acetate, or a 
mixture of the two; or cellulose acetate in acetone. No mention is 
made of any special precautions taken to ensure the stability of 
the cellulose ester base, a matter which should not be overlooked 
when treating valuable specimens. 

These solutions were employed for all kinds of materials—faience, 
glass, inlaid work, pottery, stone, wood, alabaster, amber and ivory. 
They were usually applied by means of a small camel’s-hair brush 
or a piece of pointed wood. Porous materials, such as faience or 
pottery, need thorough soaking with the solution at the broken 
edges before making the joint. Fabrics showing signs of disintegra- 
tion were sprayed with the solution. 

For details of the procedure the original must be consulted. 
There is a curious fact noted on p. 76 that silver articles sometimes 
absorbed a large amount of the solution. 


Miscellaneous Applications. 


There are a large number of miscellaneous applications of cellulose 
ester solutions which need not be entered into in detail since the 
principles involved are the same as in the applications already 
- described. 

: Cement Floors. 

Gardner finds that nitrocellulose lacquers form good priming 
coats for certain types of cement floors which have to be painted. 
By using them, waterproofing greases or other materials which may 
be in the cement are insulated from action on the enamels subse- 
quently applied. 

Imitation of Snakeskin. 

Under certain conditions of evaporation, depending partly on 
the solvent mixture, the film left by varnishes breaks up into small 
irregular polygonal areas on drying. Clément and Riviére have 
utilised this property in an ingenious way by applying such a varnish 
over a coloured base and obtaining an imitation of snake’s skin. 
A similar effect can be obtained on penholders, pencils, cartons, 
etc., and can be made permanent by application of a transparent 
~ coating.? 

Crépe Stiffeners. 

Crépe is stiffened by the application of a nitrocellulose varnish 
containing resins and black pigment. The varnish must give a 
dull, matt effect, but it must not give up pigment when rubbed or 
crushed. | 


140 Cellulose Ester Varnishes 
Hats. 


Hats are stiffened with a nitrocellulose or resin solution, which 
gives a waterproof stiffening. Straw hats are sometimes water- 
proofed with a nitrocellulose solution, but without resin, which 
would darken the colour. 


Incandescent Mantles. 


These are strengthened for transport by coating them with 
nitrocellulose slightly softened with castor oil. A high viscosity 
nitrocellulose is needed for this purpose so as to fill the mesh in 
the fabric. 


Glass. 


Glass may be coloured by coating it with transparent solutions 
coloured with various dyes, and this method is used for coating 
incandescent electric bulbs, street lamps and photographic lamps. 
A frosted appearance may be obtained by the incorporation of 
certain resins. 

Very dilute solutions of either cellulose nitrate or acetate may 
be made which leave a film so thin that it shows interference colours. 


Such films are, however, very fragile, and it is difficult to protect 


them without destroying them. 


Compressed Cork. 


Slabs of compressed cork used as bath-room mats and floor 
coverings are made by incorporating cork dust with a nitrocellulose 
solution and pressing the product in moulds into the required shape. 


Gilding Celluloid. 


Transparent celluloid may be gilded on one side by spraying 
with a solution containing fine bronze powder. Such material is 
used very effectively in the button trade. 


Imitation Gold Leaf. 


A similar solution sprayed on to glass and stripped forms a 
cheap substitute for gold leaf, and is largely employed in the 
bookbinding trade. 

Collodions. 


The term collodion is usually, though not invariably, applied to 
solutions of cellulose nitrate in a mixture composed mainly of ether 
and alcohol. Several types of collodion occur in the pharninconaem 
of which the following may be quoted :— | 


Manufacture and Application 141 
Collodion (official) B.P. synonym Contractile Collodion. 


Pyroxylin é' ie lL part 
Alcohol (90%) . 12 parts 
_ Ether (sp. gr. 0-735). : } pO O..,, 


Acetone may be used instead of ether—alcohol, but the film 
obtained is opaque. 
_ The United States Pharmacopeeia formula is :— 


Pyroxylin : ; : ; . 4 parts 
Ether. : EN | aFeage 
Alcohol . ; : = Bie 


Note that the B.P. preparation is about 2% and the U.S.P. 4%. 
The B.P. Codex has the following :— 


Collodium Acetonum. 


Pyroxylin Sw Atiai ; . 5 parts 
Clove Oil : : f : aah ae ee 
Amy]! Acetate . Ciaah Rex, 

' Benzol . : : ; : alts) ape 
Acetone to ‘ ; : , gr O0 =. 


This dries more slowly than an ether—alcohol collodion, owing to 
the presence of amyl acetate. 

Martindale and Westcott speak highly of their ‘ Collodium 
Aceto-Aethericum,” which is a 5% solution of pyroxylin in ethyl 
acetate. 


Collodium Flexile (official). 
This preparation is made to yield a flexible film, instead of the 


“contractile” film of the above-quoted official formula, by the 
incorporation of castor oil :— 


Contractile collodion ‘ : . 48 parts 
Canada turpentine . : mp + aa 
Castor oil (by weight) : kus A pak 


Various other formule are given in the Extra Pharmacopeia. 


Celloidin. 


This is a form of nitrocellulose prepared by the clarification and 
evaporation of an ether—alcohol solution. It dissolves again in 
ether-alcohol, yielding a solution of high transparency which is 
used as an embedding agent in microscopy, and also in surgery. 
It usually occurs in the form of small irregular cubes or flakes. 


_— 


142 Cellulose Ester Varnishes 


REFERENCES. 


1 §. D. Kirkpatrick, Chem. and Met. Eng., 1924, 31, 178-182. ? L. 
Clément and C. Riviére,' Chimie et Ind., 1921, 6, 283-295. 3 A. E. Ayres, 
Chem. and Met. EHng., 1916, 14, 502. 4 fo E. "Ramsabottoun Annual Report 
Advis. Comm. for Aeronautics for 1913-14, 426-432. 5 P. Drinker, J. Ind. 
Eng. Chem., 1921, 18, 831-835. ® F. W. Aston, Advis. Comm. for Aero- 
nautics Rep. and Mem., No. 585 (Feb. 1919). See PC. 10 in B.E.S.A., 
2D. 103. 7 B. D. Baker, U.S.P. 1,330,550. ® H. A. Gardner, Paint Manu- 
facturers’ Association of U.S. Educational Bureau, Circular No. 65, June 
1919. ® J. C. Wiesel, Chem. Age (New York), 1924, 32, 439. 1° H. C. 
Mougey, Paint, Oil and Chemical Review (Chicago), 1924, 78, 10-12. 11 E. M. 
Flaherty, Journ. Soc. Automotive Hng., 1924, 14, 352-353. 12 Caoutchouc 
et Gutta-Percha, 1921, 18, 10,849-10,851. 1% Joseph R. Lorenz, J. Amer. 
Leather Chemists’ Assoc., 1919, 14, 548. 14 W.K. Tucker, J. Ind. Eng. Chem., 
1921, 18, 623. 15 M. Deschiens, Chemical Age (New York), 1921, 29, 270. 
16 H. G. Crockett, ‘‘ Practical Leather Manufacture.” 17 K. L. Herrmann 
and F. J. Radel, J. Soc. Automotive Engineers, 1924, 14, 327-334. 18 G. E. 
Condé, Canad. Chem. and Met., 1924, 8, 219-220. 19 A. Lucas, ‘‘ Antiques ; 
their Restoration and Preservation > (Edward Arnold, 1924). 


Additional Reference. 


The British Engineering Standards Association publish a pamphlet 
entitled ‘‘ British Standards of Reference for Aircraft Dope and Protectin 
Covering,’’ which includes specifications for each ingredient and method of 
application. ‘To be obtained from the Secretary, 28 Victoria Street, London, 
S.W. 1, price ls. 2d., post free. 


CHAPTER IX 
MISCELLANEOUS 


Precautions Necessary in using Cellulose Ester Varnishes—Safety Precautions 
—Cause of Blooming—Humidity of Atmosphere—Measurement of 
Humidity—Effect of Raising or Lowering Temperature—Settling of 
Pigments—Cleanliness of the Work—Evaporation Losses—Analysis 
of Cellulose Nitrate Solutions; Odour, Total Solids, Nitrogen Determina- 
tions—Precipitation of Nitrocellulose with Fusel Oil (Lorenz)—Pre- 
cipitation of Nitrocellulose with Chloroform (Conley)—Precipitation of 
Nitrocellulose with Aqueous Electrolytes—lIdentification of Solvents— 
Viscosity—Analysis of Cellulose Acetate Varnishes—Solvent Recovery 
—Scientific Application of Cellulose Ester Solutions—Collodion Mem- 
branes—Interference Colours of Thin Films—Dimensions of Thinnest 
Obtainable Films—Density of Thin Films—Chromatic Emulsions— 
Transport of Cellulose Ester Solutions on British Railways. 


Precautions Necessary in using Cellulose Ester Varnishes. 
THESE may be classified under two headings :— 


(1) Precautions necessary for the safety and comfort of the 
workers. 


(2) Precautions required in order to get the best results. 


(1) The chief precautions to be taken in the employment of 
cellulose ester varnishes are those required against fire. It is an 
essential feature of these varnishes that they dry quickly, and 
that being so, the air must rapidly become laden with the inflam- 
mable vapour of the solvents unless special arrangements are 
made to renew it. When shops are being specially constructed for 
industries using these solutions, it is a comparatively simple matter 
to plan the arrangements so that fans may draw the vapours away 
from the workpeople. All the vapours are heavier than air, so 
that a fan is most efficient when placed near the ground. Direct- 
coupled motor-driven fans are not advisable on account of the risk 
of sparking, particularly if running on direct current. A belt drive 
is to be preferred. 

No general rule can be laid down as to the rate at which the 
air ought to be changed. This is governed more by the nature of 
the solvents than by the actual fire danger. In the aeroplane 
doping sheds during the war, the air was changed every two minutes, 
i.€., the total amount of air moved by the fans in two minutes was 
equal to the cubi¢ capacity of the shop.1 This is perhaps the maxi- 
mum requirement in ventilation, and it was brought about by 
earlier fatalities and illness caused by the poisonous vapour of 
tetrachlorethane. None of the solvents now in general use is as 
dangerous. Those of high vapour pressure, such as acetone, benzene 
and alcohol, require watching most closely, since the concentration 
of vapour which can be reached is naturally higher. Of these, 


benzene is most likely to give trouble, but with good ventilation it 
143 


144 Cellulose Ester Varnishes 


can be used with perfect safety. It may be remarked that if the 
ventilation is sufficiently good to prevent any feeling of discomfort 
to those working in the shop, the fire danger due to the accumulation 
of vapour is also non-existent, provided that the extraction fans 
are placed low. If the fans are placed too high, there is always a 
risk of accumulation of heavy vapours near the floor. 

Extraction fans (vacuum system) are to be preferred to fans 
blowing fresh air into the shop (plenum system). 

When much spraying work is done, it is advisable for the oper- 
atives to wear respirators, since the dried spray contains particles 
of the solid contents of the lacquers, in a very fine state of division. 

(2) The chief cause of failure met with in the use of cellulose 
ester varnishes is “‘ blooming.” This term is applied to the whitish 
deposit which sometimes appears on the surface of the work during 
drying. It is usually accompanied by brittleness of the coating, and 
when it occurs with white solutions, on which the milkiness does not 
show, it is sometimes called ‘‘ chalkiness.’”’ Another way in which 
the same fault may be manifested is by pitting of the coating. 

These faults are all due to the deposition of atmospheric moisture 
on the film during drying. If a thermometer, preferably one with 
a long bulb, is plunged into a nitrocellulose solution and withdrawn, 
it will be noticed that the mercury immediately begins to drop, 
and may fall as low as 10-15° below the temperature of the sur- 
rounding atmosphere. As a rule, before it has ceased to fall, beads 
of moisture will be seen on the film as it dries on the bulb. In fact, 
this rough test will, in the hands of an observant foreman, give 
useful information about atmospheric conditions. The cooling is 
due to the rapid evaporation of the volatile solvents, and the 
deposition of moisture to the fact that air of our climate always 
contains moisture, and that it does not hold so much when it is 
cold as when it is warm. Scientific readers must pardon the writer 
if he discusses this question in a very elementary way, as it is fre- 
quently a source of perplexity to the users of cellulose ester varnishes. 

The simplest way in which to approach the subject of water 
vapour in the atmosphere is to consider the analogy with an aqueous 
solution of a crystalline salt, say potassium nitrate. The following 
table shows how much potassium nitrate will dissolve in 100 lb. of 
water at ¢° :— 


i Lb. of Potassium Nitrate 
LO Ass ; : ae 
20° 31 
30° 45 
40° 64 
50° 86 


Miscellaneous 145 


In a similar way, we may tabulate the weight of water vapour 
which will saturate 100 lb. of air at ¢#° and normal atmospheric 
pressure 2 :— 


t° Lb. of Water Vapour 
10° ; : ; oe TT 
20° 1-48 
30° 2:75 
40° 4-89 
50° 8-68 


If we take a solution saturated with potassium nitrate at 40°, 
we see from the first table that it will contain 64 lb. of potassium 
nitrate to every 100 lb. of water. If we allow the temperature to 
fall to 30°, we see that 100 lb. of water can then only hold 465 lb. 
of potassium nitrate, so that 19 lb. of the salt must be deposited 
from the solution in the solid form. 

Similarly, if we have the air of a room saturated with water 
vapour at 40°, we see from the second table that it will contain 
4-89 lb. of water vapour for every 100 lb. of air. If we allow the 
temperature to fall to 30°, we see that 100 lb. of air can then only 
hold 2-75 lb. of water vapour, and the remainder, 2:14 lb. must 
be deposited. This moisture separates in fact as a dew or fog. 

Conversely, if we warm up a solution of potassium nitrate, 
saturated at 30°, from 30° to 40°, we can dissolve in it an additional 
19 lb. of potassium nitrate for every 100 lb. of water present. Simi- 
larly, if we warm up the air of a room, which is saturated with 
moisture at 30°, from 30° to 40°, we can make it take up an additional 
2-14 lb. of water vapour for every 100 lb. of dry air present. 

The fact expressed in the last sentence is the source of the 
misapprehension so often encountered, that “ warming the air 
dries it.” It does not. Warming the air enables it to take up an 
additional quantity of water vapour 7f it 1s maintained at the higher 
temperature, but if it is allowed to fall in temperature to its original 
state, all this extra water vapour must be deposited again. For 
example, it is of no use to draw air over steam pipes by means of 
a fan and deliver it into a cold shop. Steam pipes inside the shop 
are useful if great care is taken that all the joints are sound. A 
small leak of steam will speedily saturate the air of a room with 
moisture. Steam pipes outside the shop are only useful if the 
volume of air delivered over them, and the heat supplied to the 
air, are sufficient to warm the whole atmosphere of the shop. 

The outer atmosphere is not often entirely saturated with 
moisture. What is usually called the “ humidity ” is the proportion 
“simon as a percentage) between the amount of water vapour 


146 Cellulose Ester Varnishes 


actually present and the amount that would be present if the air 
were saturated with it. If, for example, 100 lb. of air at a certain 
temperature contains 1 lb. of water vapour, but is capable of holding 
1-5 lb., its humidity is 

 & 0 

15 * 100 = 66-:7%. 
The difference between humidity and 100%, therefore, expresses 
the drying power of the air at its existing temperature. 

Since the atmosphere always contains moisture, it is always 
possible, by reducing the temperature sufficiently, to reach a point 
at which it is saturated, 7.e., at which dew begins to deposit. This 
temperature is known as the dew point. Returning to our original 
experiment with the thermometer dipped into a nitrocellulose 
solution and then withdrawn, we see that the appearance of the 
beads of moisture on the film is due to the fact that the film has 
cooled (by evaporation of the solvent) below the dew point of the 
atmosphere. 

Measurements of humidity and dew point are useful data for 
those continuously employing cellulose ester solutions. They can 
be simply determined by means of an apparatus known as the 
“wet and dry bulb thermometer,” which consists of two identical 
thermometers mounted side by side on the same stand. One of 
the bulbs—the ‘“‘ wet bulb ’—is covered with a piece of muslin, 
which is connected by a few strands of cotton to a small reservoir 
containing water. The capillary action of the cotton keeps the 
muslin continually damp with water. When the air is very dry, 
the moisture evaporates rapidly from the muslin, and lowers the © 
temperature of the mercury. If the air is damp, evaporation of 
the water is hindered, and the lowering of temperature is less. 
Hence the difference between the temperatures recorded by the 
two thermometers is governed by and measures the humidity of 
the surrounding air. Tables, originally complied by Glaisher,® are 
available from which the humidity of the air can be read directly 
from the temperatures of the wet bulb and the dry bulb. 

A modern variation of the wet and dry bulb thermometer is 
made in which the thermometers can be whirled on an axle, so that 
the air is in rapid motion past the bulbs. Instruments known as 
hygrometers are also made which indicate the humidity directly 
on a dial. They depend on the influence of atmospheric moisture 
on the length of animal fibres such as horse-hair. 

The following monthly averages of atmospheric temperature 
(dry bulb), wet bulb temperature, humidity and dew point may be 


Miscellaneous 147 


of interest. They were taken outside the writer’s laboratory 
(Suffolk) at mid-day on working days, and show the kind of variation 
which occurs between summer and winter weather :— 





1924. Dew Humidity 
point. ss 
Eee 60-0° F. 75-6 
OS eae 52:6 64-2 
September .............5. 52-2 67-4 
SS re 49-4 74-7 
PRovemiber: ..0.....-...+. 42-5 75-2 
se 40:7 76:8 


The chief point to observe is that the dew point in the open air 
is lower in winter than in summer, and this is the temperature at 
which the atmosphere begins to deposit dew. Supposing that, 
without any change in the actual weight of the water vapour content, 
the air defined in the above table were drawn into a shop at a 
temperature of 55° F. The dew point in July averaged 60° F.; 
therefore this air at 55° F. would be below its dew point and would 
become foggy and deposit dew. The other five samples would 
remain clear. 

The usual working temperature of a shop is about 65° F., and 
this is a fairer temperature to take. If the above six samples of 
air were drawn, without change of water content, into a shop at 
65° F., what effect would it have on their drying properties? This 
can easily be ascertained from Glaisher’s tables. Since the water 
content has not changed, the dew point of each sample must be 
unchanged also. We have to find therefore what would be the 
humidity of six samples of air, whose dry bulb temperature was in 
each case 65° F., but whose dew points were 60-0, 52-6, 52-2, 49-4, 
42-5 and 40-7 respectively. The answers are given in the following 
table :— 





1924. Humidity if at 65° F. 
NE irs ay a sinengiobesakgnnaninns 84% 
MIE ie wentasecrsava setae 64 
BODUGIADOT 55. sscyiesvedacsees . 63 
SOB ois hncsasedewigtns 57 
PHOVGIMDG? So accocssvecccd cess 44 
- PAGCOR a ooo sd dads bo vhs eds 41 


It will be noticed that the humidity of the July air has been increased, 
that of August has hardly changed, while the average air of the 
other four months has all been reduced in humidity by bringing it 


148 Cellulose Ester Varnishes 


to the uniform temperature of 65° F. The content of water vapour 
has not been altered, The change has occurred in the proportion 
between the actual water content, and the water that the air could 
hold if saturated; in other words, the drying power of the air has 
been changed, although its water content has not. 

It follows from this that if work is carried on in a shop main- 
tained at a uniform temperature throughout the year, trouble with 
blooming is more likely to occur in summer, when the dew point of 
the outer air is high, than in winter, when it is low. In practice, 
conditions are not quite so simple as they have been assumed to be 
in this example. For example, the presence of human beings in a 
room alters the humidity conditions, owing to the moisture in 
expired air; but the main conclusions hold. 

Solutions sold by manufacturers as drying bright contain 
sufficient high boiling solvent to prevent blooming under all usual 
atmospheric conditions, but when consumers ask for specially 
quick-drying solutions, blooming becomes a frequent problem. 

It is evident that there are three ways of preventing the deposition 
of atmospheric moisture on the film :— 


(1) Drying the air which is being drawn into the shop by 
the exhaust fans. 

(2) Checking the rate of evaporation, so as to diminish the 
cooling of the film. 

(3) Carrying out the drying operation in a chamber at a 
slightly higher temperature than the work-room. 


A combination of (2) and (3) is the usual remedy for blooming. 
Drying the air, the first remedy, is a rather more complicated process — 
than most solution-users care to undertake. It may be carried out 
by passing the incoming air over refrigerating coils, when the 
moisture in the air is deposited on the pipes as ice, or by drawing 
the air through a drying tower fed with sulphuric acid. Sometimes 
trays of calcium chloride in lumps are placed in the drying chamber, 
thereby combining methods (1) and (3). If the work, after having 
been sprayed or dipped, is promptly transferred to a warm com- 
partment, at a temperature of 80° to 90° F., solutions made with 
ordinarily volatile solvents will usually dry bright. In such a 
compartment, the rate of evaporation can be checked by means of 
ventilators, thus applying method (2). The air under these con- 
ditions becomes highly charged with vapour, and precautions 
against fire must be rigorously enforced. 

Varnishes containing high-boiling solvents do not often give 
trouble through blooming, but it must be remembered that the 


% 


Miscellaneous 149 


high-boiling solvents are the expensive ones, and keen competition 
will compel manufacturers to reduce the percentage of these solvents 
to the lowest limit. 

Setiling.—The rate of settling of the pigment in a well-made 
cellulose ester varnish is extremely slow, but nevertheless it does 
occur when the pigment is heavy or present in large amount. Such 
varnishes should therefore be well stirred before use, and should 
not be stocked for long, if this can be avoided. If the varnish is 
allowed to settle, the top layers will be deficient in covering power, 
and the bottom layer will give brittle or rough coatings. 

Cleanloness.—It is particularly necessary that surfaces, especially 
metallic surfaces, to be coated with either a cellulose nitrate or 
cellulose acetate varnish, should be quite clean. Grease or oil is 
specially to be avoided, as it prevents proper adhesion. Washing 
the surface with petroleum spirit, benzol or toluol is recommended 
when practicable, but the surface must be quite dry again before 
the first coating is applied. 

Evaporation Losses.—It is the custom in many factories using 
cellulose ester solutions to distribute the solutions about the shops 
in small vessels or containers, for the conveniente of the work- 
people. Unless these can be closed at night with an air-tight cover, 
they should always be emptied back into the main storage vessel. 
Otherwise a great deal of solvent is wasted, some of the varnish is 
spoilt, the air of the shop is needlessly contaminated with solvent 
vapours and the fire risk while the shop is unoccupied is unneces- 
sarily increased. 

Analysis of Cellulose Nitrate Varnishes.—In view of the fact that 
very few of the ingredients of cellulose ester varnishes are, or even 
approximate to, chemically pure substances, it is evident that the 
deduction of a formula by the analysis of a sample is always a matter 
of great difficulty, and sometimes an impossibility. In fact, as a 
rule more can be learned about a sample in a short time by 
close observations of its behaviour on filming, its odour and its 
price, than can be learned in the same time from its chemical 
behaviour. ; 

Certain methods are available, however, and yield results which 
are of considerable assistance. 

(1) Odour.—The constituents of the solvent can often be identi- 
fied by the smell of the sample. Amyl acetate is unmistakable, 
but may mask butyl acetate. Acetone, benzene, wood spirit and 
ethyl acetate can usually be recognised, even when present together. 
Petroleum spirit is less easily distinguished and ethyl alcohol is 
difficult to detect. There are, of course, considerable personal 


150 Cellulose Ester Varnishes 


differences in the acuteness of the sense of smell and the power of 
distinguishing the constituents of mixtures. 

(2) Total Solids—The percentage of total solids is determined 
by the evaporation of a small weighed quantity to constant weight. 
The solution should be spread out in as thin a film as possible. 
Conley 5 recommends using just enough of the solution to cover 
the bottom of the weighing bottle, but this limits the quantity 
rather stringently. On the other hand, weighing on a clock glass 
is inadmissible on account of the rapid loss of solvent during weighing. 
A preferable method is to weigh the sample in a wide-mouthed bottle 
or sample tin; the rounded bottom of a stout weighed glass tube is 
then dipped into the solution and withdrawn, rotating it in the 
fingers if there is a tendency to drip. The sample is weighed again, 
and the difference in weight is the amount transferred to the outside 
of the tube. The latter is kept in a warm place until free from the 
smell of solvent and is then weighed at intervals until of sufficiently 
constant weight. The appearance and manner of drying of the 
film can be very conveniently studied in this way, and the coating 
is easily removed as a thimble by putting the tube in hot water 
for a few seconds. 

Nitrogen Determinations.—Writers on the subject do not appear 
to have realised the value of a nitrogen determination in a portion 
of the film obtained from a total solid determination of a cellulose 
nitrate solution. This is best carried out by the Schultze-Tiemann 
method on a weighed quantity of about 0-3 gramme of the sample. 
[It will probably be noticed that there is a distinct odour of solvent 
when this piece is boiled in the flask—a sign that the estimation of 
solid content has given a higher figure than the true one.] From 
the nitrogen figure it is possible to obtain immediately an approxi- 
mate indication of what proportion of the solid content is noé nitro- 
cellulose. The nitrocelluloses used in varnish manufacture usually 
contain from 11-7 to 12-2% of nitrogen. Hence if the nitrogen is 
much lower than this, there must be an appreciable quantity of 
other ingredients and the percentage of nitrocellulose in the film 
will be roughly 

a 


O 
75 X 100% 


of the weight of the film, x being the determined percentage of 
nitrogen. 

If the film is transparent and nearly colourless, the other con- 
stituent may be castor oil, which imparts its characteristic odour 
to the film and makes it supple. Ifthe film is transparent, somewhat 


Miscellaneous 151 


dark in colour and hard, resins are indicated. If opaque, there are 
probably pigments present and a determination of ash should be 
made to compare with the value calculated from the percentage 
of nitrocellulose. The ash may be analysed by the ordinary methods 
of inorganic analysis. The presence of oil in the film is often shown 
by drops floating on the surface of the liquor remaining in the flask 
after the completion of the nitrogen determination. Resins, oil 
and pigment may all be present together, or any two of them, and 
the problem becomes complicated. Extraction with an organic 
solvent which does not dissolve nitrocellulose, such as chloroform 
(best), benzene or petroleum spirit, will often take out the resins 
and/or oil, leaving pigment and nitrocellulose together. A fresh 
nitrogen determination at this stage will check the loss of weight 
of the film and the weight of the oil extracted. When weighing 
an oil extract which is being taken to approximately constant 
weight, it is usually advantageous to employ a small porcelain 
dish resting on a watch glass or small clock glass, so that any oil 
creeping over the side of the dish is not lost. 

Precipitation with Fusel Oil—For a mixture containing nitro- 
cellulose, resin and pigment, amyl acetate, amyl alcohol, methyl 
alcohol and petroleum spirit, Lorenz* recommends a different 
method, which has its advantages. To 100 c.c. of the varnish are 
added gradually with continuous stirring 200 c.c. of fusel oil. The 
nitrocellulose is precipitated as a voluminous gel. Fifty c.c. of the 
clear liquor are drawn off and saponified with standard alcoholic 
potash. If the only ester present is amyl acetate, the amount in the 
50 ¢.c. is one-sixth of that originally present in 100 c.c. of the 
varnish. 

[If butyl or ethyl acetate is present as well as amyl acetate, it 
is only possible to obtain the total percentage of combined acetic 
acid in the original varnish, a figure that may be quite useful.] 

The remaining 250 c.c. from the previous determination are 
transferred to a tared filter and washed with warm fusel oil until 
no more solid matter is extracted. The object of this is to extract 
all the resins (of which amyl alcohol is perhaps the most general 
solvent), leaving the nitrocellulose on the filter-paper. An aliquot 
portion of the filtrate is taken to dryness to give the percentage of 
resins. Lorenz remarks that ‘‘ the nature of the gum resins and 
camphor which constitute the residue may be determined by the 
usual methods.” If camphor is present, the writer would much 
prefer to get rid of it before weighing the resins, since the amount 
of camphor left after evaporating to dryness 150 c.c. of fusel oil (as 
Lorenz recommends) would be much less than the camphor originally 


152 Cellulose Ester Varnishes 


present, because of its high volatility. The writer would suggest 
moistening the residue of resins with toluene, and taking to dryness 
again, repeating this operation until sufficient constancy of weight 
is reached. The presence of small amounts of camphor in varnish 
samples is a much more awkward analytical problem than the 
estimation of camphor in solid celluloid. The only safe method of 
camphor estimation is by the polarimeter (which, of course, fails 
when optically inactive camphor is used), and it is not easy to get 
from a sample of varnish a solution of the camphor (in a pure solvent) 
of sufficient concentration to give an accurate polarimeter measure- 
ment of its optical activity. The writer would prefer to proceed 
on the following assumptions :— 


(a) If camphor is detected qualitatively, and the nitrogen 
content of the nitrocellulose is not greater than 11%, the 
varnish has probably been made from ordinary celluloid, and 
should be matched by the use of ordinary celluloid based on 
the nitrocellulose content of the latter. 

(b) If the nitrogen content of the nitrocellulose is about 
12% and camphor is present, the varnish has probably been 
made from waste cinema film, and should be matched by the 
use of cinema film based on its nitrocellulose content. 


The assumption underlying this procedure is that camphor is 
not likely to be added to a varnish made up from fresh nitrocellulose, 
and its presence usually indicates the use of a form of celluloid 
scrap. Admittedly exceptions occur. In any case, the proportion 
of camphor is not likely to exceed 25% of the weight of nitrocellulose. 

Returning to the analytical scheme of Lorenz, the mass on the 
filter-paper now consists of nitrocellulose and adhering fusel oil, 
together with pigment. It is washed with a mixture of equal parts 
of acetone and amyl acetate, which dissolves the nitrocellulose, 
and the solution passing through the filter is collected. On 
evaporation to dryness of the whole of this (or an aliquot portion), 
the nitrocellulose is obtained and can be weighed. The writer’s 
experience is that the weight of nitrocellulose so obtained is likely 
to be too high. (See p. 154.) 

The residue on the filter-paper is the pigment, the weight of 
which is obtained by drying to constant weight at 100° and deducting 
the weight of the filter. It is the writer’s opinion that with many 
of the finely-ground pigments used in present-day lacquers, a certain 
amount would pass through the filter with the nitrocellulose, so 
that the figure would be only approximately correct. Some method 
such as this, however, is the only practicable one if the pigment is 


Miscellaneous 153 


some form of carbon, since it cannot be ignited without loss. If 
the pigment is a permanent oxide, such as zinc oxide, it is better 
to determine pigment by ignition of the film obtained in the estima- 
tion of total solid contents, as already described. 

Petroleum spirit is determined by adding 75 c.c. of concentrated 
sulphuric acid to 50 c.c. of the solution in a graduated cylinder, 
stirring vigorously. After several hours, the petroleum spirit, 
which is the only ingredient insoluble in the acid, will have risen 
to the top and the volume may be read. 

Lorenz estimates the amounts of methyl and amyl alcohol by 
distilling 100 c.c. of the original sample, using a fractionating column, 
and assuming that the volume of distillate coming over below 75° 
is equal to the content of methyl alcohol. Of the four liquid 
constituents of the mixture, amyl acetate, amyl alcohol, methyl 
alcohol and petroleum spirit, three have thus been determined, and 
the amyl alcohol is calculated by difference. 

Conley’s Scheme of Analysis.—Conley 5 abandons the method of 
precipitation by fusel oil, since some nitrocellulose remains in 
solution. He prefers to measure out 10 c.c. of the solution, and 
precipitate by the gradual addition of chloroform, with continuous 
shaking. If no resins or pigments are present, the precipitate is 
pure nitrocellulose, and may be filtered off, dried, and weighed. If 
resins are present in considerable amount, the first precipitate of 
nitrocellulose will be contaminated with resin. It is then necessary 
to decant the clear liquor as completely as possible, to redissolve 
the nitrocellulose in the smallest possible quantity of solvent, and 
to reprecipitate with chloroform. Conley does not suggest a solvent 
for this purpose, but the writer would suggest a low-boiling solvent 
such as acetone, so as to diminish the risk of contaminating the final 
precipitate with a high-boiling solvent such as amyl acetate. 

Resins or oil are estimated by evaporation to dryness of an aliquot 
portion of the combined filtrate. Castor oil is recognised by its 
smell and its relative insolubility in petroleum spirit. It is almost 
impossible to identify the resins obtained in this way, since their 
solubility is frequently altered by the treatment they have received. 

The writer’s experience is that no method of precipitation of 
nitrocellulose by organic solvents is cleanly or easy to carry out, 
although in view of the fact that the analysis of a nitrocellulose 
varnish is at best a very approximate affair, the method is probably 
good enough. From the researches of Knoevenagel and his 
collaborators on the swelling of cellulose acetate, it is probable that 
the precipitated nitrocellulose contains absorbed solvent, in some 
kind of equilibrium with the surrounding liquid, and it is difficult to 


154 Cellulose Ester Varnishes 


remove this in reasonable time by drying. If a precipitate so 
obtained, after having been taken down to approximately constant 
weight, is transferred to a test-tube and boiled with a little water, 
there is an immediate odour of solvent, showing that the nitro- 
cellulose is not pure. 

Precipitation with Aqueous Electrolytes—The following method 
is much cleaner, and has been employed in the writer’s laboratory 
for several years, if a more accurate determination is necessary. 
A film is first obtained directly from a weighed or measured quantity 
of the original solution, as in the determination of solid content. 
The amount of film should be enough to yield about 0-5 gramme of 
nitrocellulose. If oils or resins are present, they are extracted 
under reflux or in a Soxhlet, with chloroform or any other appropriate 
solvent. If they are not present, this step is unnecessary. The 
_ film is then dissolved in 50 c.c. of acetone, and diluted with aqueous 

alcohol (50%). The quantity cannot be stated definitely, since it 
differs according to the solubility of the nitrocellulose, but it must 
be sufficient to convert the solution to an opalescent suspensoid 
colloid, blue by reflected light and red by transmitted light. Such 
a solution would pass unchanged through filter-paper. On adding 
to it a dilute aqueous solution of an electrolyte, such as 2% 
ammonium chloride, it is coagulated to a manageable precipitate. 
Here also the required conditions are variable. Sometimes it is 
better to precipitate hot, sometimes cold. A little experience is 
the best guide. Under favourable conditions, the precipitate is as 
easy to manipulate as silver chloride. The liquid is brought to 
the boil, allowed to settle, and filtered off through a Gooch crucible 
under suction, the first few washings being decanted off. Washing 
is complete when the filtrate contains no chloride. The precipitate 
is dried in the usual way at about 40°, and when constant weight 
is obtained, it may be washed directly into the Schultze-Tiemann 
apparatus for nitrogen determination. The greater purity of the 
nitrocellulose so obtained is shown by the fact that it always gives 


a higher nitrogen content than those obtained by precipitation with | 


an organic liquid, and the apparatus is left quite clean. 

This method is a modification of one first suggested by Dubovitz,® 
who precipitates an acetone solution of nitrocellulose directly with 
a solution of ammonium chloride. The modification is a little more 
elastic, and the precipitated nitrocellulose is less likely to contain 
enclosed impurities. The hot aqueous solution of acetone and 
alcohol is very efficacious in keeping camphor and other organic 
solvents in solution. 


Idenittfication of the Solvents —Conley recommends dry distillation 


# 


Miscellaneous 155 


up to 120° of 100 c.c. of the varnish from a distilling flask heated 
in a paraffin bath. Above 120°, water is added, and a steam distil- 
lation is substituted. The product of dry distillation will contain 
all the low-boiling solvents present, e.g., methyl and ethyl alcohols, 
acetone, methyl ethyl ketone, ethyl acetate, benzene and part of 
the petroleum spirit. The product of the steam distillation will 
contain, ¢.g., amyl acetate, butyl acetate, higher acetone oils, and 
the remainder of the petroleum spirit. 

The rigid separation and identification of these in the limited 
time available in a technical laboratory is impossible. The low- 
boiling solvent should be fractionated, and a selection from the 
following tests applied to the fractions: odour, specific gravity, 
miscibility with water, miscibility with salt solution, miscibility 
with concentrated sulphuric acid, iodoform reaction, reaction of 
immiscible liquids with nitric acid. The product of the steam 
distillation, after salting out dissolved solvents from the water 
layer, may be similarly treated. From the indications obtained it 
is usually possible after some experience to match fairly closely the 
composition of the solvent mixture. 


Viscosity. 

This is the most important property of all. It should be 
measured by a method giving results comparable with those of 
other investigators, and the writer strongly recommends the use 
of the falling sphere viscometer as standardised by Gibson and 
Jacobs.? The instrument should be calibrated against a standard 
material such as glycerine, and the result expressed either in absolute 
C.G.8. units, or in a unit easily convertible into the scientific unit. 
A modification of the falling sphere viscometer due to Mardles 
can be used for opaque solutions in which a weight is attached to 
a fine wire on the other end of which is a counterpoise.* The wire 
is hung over a low-friction pulley and the weight is allowed to fall 
in the solution. The rate of fall is then measured by the rate of 
rise of the counterpoise. The B.E.S.A. specifications favour the 
Ostwald viscometer. Their standardisation of the temperature 
of 25° for viscosity determinations is to be commended. Although 
this temperature is a little higher than that at which most solutions 
are used, it is one that can be maintained in a thermostat in hot 
weather without difficulty. 


Analysis of Cellulose Acetate Varnishes. 


There is no literature on the subject of the analysis of cellulose 
acetate varnishes, probably because there has been very little need 


156 Cellulose Ester Varnishes 


for such analyses up to the present. The writer suggests the 
following scheme as a working basis :— 


(a) Determination of solid content by filming-off a weighed 
quantity of solution. The solid content consists of cellulose 
acetate and the softeners, such as triphenyl phosphate, tri- 
acetin, benzyl alcohol, acetanilide. Extraction of the film 
with low-boiling non-solvents such as ethyl ether. Weight of 
cellulose acetate remaining, and determination of acetyl group 
if of interest. Resins are not found in cellulose acetate varnishes. 

(b) Identification of the softener by precipitating a larger 
quantity of the dope with ether, filtering off the precipitated 
cellulose acetate and evaporating to approximately constant 
weight. Test the residue for organic phosphate and for the 
phenyl, glyceryl, benzyl, acetyl and amino-radicals. Quanti- 
tative determination is likely to be difficult or impossible. 
Estimations of phosphate and of nitrogen are most likely to 
yield useful information. 

(c) Dry distillation to 120° followed by steam distillation, 
as in Conley’s scheme for nitrocellulose varnishes, should give 
much information about the volatile solvents. Tetrachloro- 
ethane is not likely to be found in a modern acetate varnish, 
but is easily recognised by the smell. The solvents for which 
a look-out should be kept are acetone, methyl ethyl ketone, 
alcohol, benzol, methyl acetate. Fractionation of the dry 
distillate and miscibility tests of the fractions would show the 
presence of benzol, acetone and methyl ethyl ketone (note 
smell and boiling point). Quantitative determinations are not- 
likely to be of much use, but a saponification to determine ester 
(methyl acetate) and an acetylation to determine free hydroxyl 
(ethyl! alcohol and possibly methyl alcohol) may be worth while. 


Addendum to Analysis. 


In converting proportions by weight to proportions by volume, 
or vice versa, it is necessary to know the specific gravity of the 
solution. ‘There is no advantage to be gained by carrying out this 
determination in a pyknometer with the precautions usual in 
scientific work, and it is difficult to do so on account of the high 
viscosity and volatility of the solution. A better way is to weigh 
25 or 50 c.c. in a tared graduated cylinder. The accuracy obtained 
in this way is much greater than the probable accuracy of any 
determinations of individual solvents which may be attempted. 

The cultivation of a sense of proportion is very necessary in 


Miscellaneous 157 


technical analysis if high accuracy is unattainable, and will often 
lead to much saving of time. The accuracy of an analysis is limited 
by that of its least accurate step, and it is to the improvement of 
the latter that care and attention should always be given. 


Solvent Recovery. 


In view of the fact that a large proportion of the weight of a 
cellulose ester varnish consists of volatile liquids which evaporate 
from the coating in the course of drying, there would appear to be 
considerable scope for the application of processes of solvent 
recovery. Actually, however, there are few industries using these 
varnishes in which solvent recovery has been installed. Perhaps 
the chief reason for this is that the consumption of cellulose ester 
varnishes is spread over a large number of small users, and solvent 
recovery is much more profitable for large quantities of solvent 
than for small, on account of the high overhead charges on small 
plants. Another factor is the considerable range of different solvents 
employed, and the difficulty of ascertaining the exact composition 
of a mixed recovered solvent—knowledge which is essential before 
the solvent can be employed again for a similar purpose. However, 
very great strides were made in the theory and practice of solvent 
recovery during the war, and it is possible that there may be 
considerable developments during the next few. years. 

The processes of solvent recovery fall into three principal 
classes :— 


(1) Absorption of the solvent vapour in a suitable liquid or 
combination of liquids. 7 

(2) Adsorption of the solvent vapour on the surface of solids. 

(3) Direct condensation of the vapours, using refrigeration. 


The third method is only economical when the concentration of 
the vapour is very high, or in certain processes where partial vacuum 
is employed to hasten evaporation. Since the vapour coming from 
work-rooms in which cellulose ester solutions are used is necessarily 
dilute, this method need not be further considered. 

The first method is the one which has been used to the greatest 
extent in industry, and usually resolves itself into a search for a 
suitable absorbent. When alcohol vapours alone have to be cap- 
tured, as in the final stages of manufacture of certain soaps, water 
is an excellent absorbent. In the Chardonnet process of artificial 
silk manufacture, in which an ether—alcohol solution of nitro- 
cellulose is formed into threads, large quantities of ether—alcohol 


158 Cellulose Ester Varnishes 


are set free, and much of the earlier work was carried out in con- 
nexion with this industry. Although water is an excellent absorbent 
for alcohol, it is not so for ether, and the only absorbents which 
were at all successful in capturing both solvents on a large scale 
before the war were amy] alcohol, and, better, sulphuric acid. When 
the manufacture of R.D.B. cordite, which used ether-alcohol in 
place of acetone as the solvent, was undertaken on an enormous 
scale in Great Britain during the war, the question of recovery 
became pressing, and absorption in sulphuric acid was carried on 
for some time at Gretna. A peculiarity of this process is that ethyl 
alcohol reacts with sulphuric acid to form ethyl hydrogen sulphate, 
which reacts reversibly with either alcohol or water to form ether 
or alcohol respectively, according to the equations :— 


(1) C,H,-H:SO, +- C,H;-OH =— C,H,-0-C,H; - H,SQ,. 
(2) C,H,-H-SO, + H,O — C,H, OH + H,S0,. 


Hence the proportions of ether and alcohol in the recovered solvent 
altered according to the concentration conditions when the absorbent 
was distilled. The equilibria in this process were studied by Masson 
and McEwan.® The disadvantage of this process was the excessive 
corrosion of plant brought about by the sulphuric acid. 

The use of cresol as an absorbent was patented by Brégeat in 
1917,1° and this substance was found to be an excellent absorbent 
for both ether and alcohol. A large experimental plant working 
on this system was erected at Gretna, and a complete plant for 
dealing with the whole of the vapour from the cordite stoves was — 
erected, but not used owing to the termination of the war. In 
this plant the vapours were drawn by immense fans through Whessoe 
scrubbers through which cresol was circulated in counter-current. 
The saturated cresol was steam distilled to remove the alcohol and 
ether, and was sent back, after being cooled, to the scrubbers. The 
crude distillate was distilled over soda to remove cresol and frac- 
tionated for the recovery of alcohol and ether. Some of the 
information obtained at Gretna in the design and working of the 
plants has been published in the Technical Records of the Ministry 
of Munitions.14 

Since cresol is miscible in all proportions with the common 
cellulose ester solvents, it is probable that it would, under favourable 
conditions, effect a good recovery of the mixed vapours with which 
it would have to deal. According to Drinker,’ methyl, ethyl and 
amyl alcohols and acetates, acetone, benzene, toluene, xylene and 
chloroform can all be recovered in satisfactory yield, but it is not 
stated how large a plant is necessary before a satisfactory return 


Miscellaneous 159 


on the capital outlay is assured. The rapid fluctuations in the 
prices of solvents since the war do not make the problem any 
easier. 

The adsorbents used in the second method are activated charcoal 
and silica gel, the former, as its name implies, a specially prepared 
carbon, and the latter a form of precipitated silica. War-time 
experience with charcoal used in the box respirator as an absorbent 
proved the efficacy of this substance as a means of removing vapours 
of substances of high molecular weight from air, and its use for 
solvent recovery is based on this absorbent power. Silica gel 
possesses similar properties, and the rival claims of these two 
absorbents are still under discussion. Neither appears to have been 
used yet in this country for the recovery of solvents from varnishing 
‘operations, but each promises to be a serious rival to the processes 
using liquid absorbents. 


Scientific Applications. 


Collodion Membranes.—The properties of collodion membranes 
have interested scientific investigators ever since the middle of the 
nineteenth century, chiefly on account of their restricted permeability. 
There is a useful historical summary in Worden’s “‘ Nitrocellulose 
Industry.” #2 The interest in this subject has increased rather 
than diminished during the last few years, owing to the prominence 
which the subjects of permeability and osmotic pressure have 
assumed in biology, medicine and chemistry. The writer is not 
qualified to give an extended account of this work, which would in 
any case be out of place in this volume, but a short reference should 
be made to it in so far as cellulose nitrate solutions are concerned. 

Collodion membranes are made by allowing a solution of nitro- 
cellulose in ether alcohol to evaporate, leaving a film. If, for example, 
a test-tube is dipped into such a solution, withdrawn and rotated, 
the solution rapidly thickens to a firm jelly and will in time dry up 
to a tenacious film. Before that stage is reached, however, the 
tube is dipped into water, which stiffens or “sets ” the film, which 
can then be withdrawn from the outside of the tube as a thimble or 
sac. Such membranes may be made, by methods to be mentioned 
later, to vary considerably in permeability and can be used to filter 
off substances which would pass through any ordinary filtering 
medium such as filter-paper. To this process Bechhold applied the 
word. “ ultra-filtration.”’ The collodion membrane may be made 
to withstand somewhat high pressures. 

Walpole has published a considerable amount of information on 
this subject.14 The use of these membranes depends on the 


160 Cellulose Ester Varnishes 


different porosity of the gel of the membrane to molecules of different 
sizes. If, instead of using the membrane as a filter, the filtrate side 
of the membrane is bathed in a solution other than the filtrate, the 
process becomes dialysis, and Walpole finds many advantages in 
dialysis under pressure, using collodion bags which will stand a 
pressure of 2—3 atmospheres. Following earlier work by Martin 
and Cherry, he found that the toxins of both tetanus and diphtheria 
were held back by collodion membranes, the filtrates or dialysates 
of the sera being free from them. 

Walpole found that collodion membranes were more uniform 
than parchment, and proposed to standardise them. The pro- 
perties of these films depend on a large number of factors, beginning 
with the origin of the cellulose and its treatment before nitration, 
and ending with the pressure applied to the membrane when it is 
in use. They can, however, be characterised by two factors :— 


(1) The weight of dry nitrocellulose per square centimetre 
of the film. 

(2) The wetness of the film, which is defined as the ratio of 
the wet weight of the film (after being soaked in distilled 
water until it is saturated) to the weight of nitrocellulose which 
it contains. 


The wetness of the film is a measure of its porosity,.and it 
diminishes as the time allowed for evaporation in the preparation 
of the membrane increases. Thus the permeability of a film which 
has dried up completely before it is detached from the glass support 
is very low. 

The weakness of this method of characterisation seems to the 
writer to be that a film containing comparatively few large pores, 
and another containing comparatively many small pores, might hold 
the same amount of water, 7.e., be equally wet, and yet differ con- 
siderably in permeability. Films made from the same solution of 
nitrocellulose with different times of drying might give repro- 
ducible results if standardised in this way, but it seems doubtful 
whether the results could be repeated if the films were made from 
an ether—alcohol solution of an entirely different sample of nitro- 
cellulose. However, Walpole obtained very interesting results, 
and gives the characteristic factors for films which would pass all 
simple molecules, but retain quantitatively all antigens; he also 
gives data on the permeability of the membranes to various enzymes. 

Keeble 15 found that the hormone of the sensitive plant Mimosa 
pudica, by which the excitation of the tissue is transmitted, will 
diffuse across a film of collodion without losing its potency. 


Precautions Necessary 161 


W. Brown !¢ has studied the technique of membrane preparation. 
‘The permeability of the film may be altered by soaking it in various 
mixtures of water and alcohol, and Brown suggests an alcohol index, 
denoting the strength of alcohol with which a film comes into 
equilibrium. Membranes can be made that will hold back so 
simple a molecule as copper sulphate, or that will pass so large a 
molecule as aniline blue. He expresses the process of membrane 
formation in a generalised form. If we have a “ restraining liquid ”’ 
A which is not imbibed by the membrane, and a “ swelling liquid ”’ 
B which is strongly imbibed by the membrane, then if A and B are 
miscible in all proportions, it is possible to obtain grades of per- 
meability by steeping the films in mixtures of A and B, subsequently 
washing them in A. The higher the proportion of B in the steeping 
liquid the greater the permeability obtained. In this way, Brown 
has prepared membranes of cellulose nitrate, cellulose acetate, agar 
and gelatin. } 

| Farmer 1’ also found that the permeability varied according to 
the time of drying, and Eggerth 18 that it depended on the proportion 
of alcohol in the original collodion. Gans found that the per- 
meability could be varied by the use of acetic acid. Looney *° 
obtained more flexible membranes by using a proportion of ethyl 
acetate in the collodion, but this increased flexibility would probably 
not be permanent. 

These researches are recalled by the work of Knoevenagel (q.v.) 
on swelling, which is directly related to them. 

Wegelin 2! used solutions of nitrocellulose in acetic acid instead 
of ether—alcohol for the preparation of the membranes, and found. 
that the film from a 7-5% solution was ten times as porous as a 
film from a 15% solution. Experimental details are given. The 
membranes were used for the dialytic purification of various well- 
known inorganic colloids, and he suggests using them for the measure- 
ment of the size of particles in colloidal solutions and for the purifica- 
tion of precipitates not easily washed by ordinary methods. 

S. L. Bigelow ?? in 1907 studied the permeability of collodion 
membranes, gold-beaters’ skin and parchment paper, to pure liquids, 
and concluded that the rate of passage of the liquids through them 
was governed by the same laws as the flow of liquids through 
capillary tubes. Duclaux and Errera 2? in 1924 found that the 
velocity of filtration of water, aqueous solutions and certain organic 
liquids through collodion membranes was approximately propor- 
tional to the viscosity, and thus support Bigelow’s conclusions. 
Bartell and Carpenter,?4 however, found examples of anomalous 
gamed which they attributed to the influence of the electrical 


162 Cellulose Ester Varnishes 


properties of the membrane system. It is of interest to note that 
they prepared their films on the surface of mercury. Preuner 
and Roder 25 also found abnormal osmosis, which they explained 
by the existence of a potential difference between the walls of the 
membrane, differing according to the concentration of the solutions. 

Duclaux ** has patented the preparation of continuous lengths 
of membranes for ultra-filtration by spreading a solution of a cellulose 
ester on a moving strip of fabric and coagulating the film in a suitable 
liquid. 

Various papers have been published during the last few years 
dealing with the preparation of membrane filters, and their employ- 
ment in analytical operations, among which those of Zsigmondy and 
Bachmann,?? Bachmann,?§ Jander,2® Kling and Lassieur,®® and 
Bechhold and Gutlohn *! may be mentioned. 


Miscellaneous Scientific Uses. 


Several investigators have interested themselves in the properties 
of extremely thin films of collodion, but the materials used in their 
experiments are sometimes described somewhat vaguely. Wood 
prepared collodion films of soap-bubble thickness by diluting 
‘ordinary collodion ’’ with about ten parts of ether, pouring the 
mixture on a glass plate and draining it off immediately. If a 
square is ruled on the glass with the point of a sharp knife, and the 
plate is lowered into water, the square piece of film may be floated 
off. It may be picked off the water by lowering on to it the well- 
greased edge of a cylindrical support such as the bottom of a 
cylindrical lamp glass. . 

The same observer noticed *% that a thin collodion film deposited 
on a bright silver surface shows brilliant colours in reflected light, 
which, however, were not obtained if the commercial ether in the 
collodion were replaced by pure ether. Similar films on glass did 
not give the same effect. This phenomenon attracted the attention 
of the late Lord Rayleigh.*4 

Boys *° used a solution of celluloid in amyl acetate to abbade 
permanent iridescent films. He quotes directions given by Glew. 
Amy] acetate is boiled for a few minutes to remove all moisture 
from it. It is then used to make up a solution of celluloid in the 
proportion of 1 gramme in 14-6 c.c. A drop of this solution is 
allowed to fall on the surface of clean cold water in a large basin, 
and the amyl acetate is allowed to evaporate. The film may be 
lifted from the water on a wire ring, and becomes more brilliant as 
it dries in the air. Glew states that the films may be thrown into 


Precautions Necessary 163 


nodes and loops by sounds of short wavelength; the effects may be 
studied by reflecting sunlight from the film on to a sheet of white 
card. Whistling or singing near the film then causes a charming 
combination of motion and colour on the screen. 

Barton and Hunt °° describe how extremely thin films of “ cellu- 
loid ’’ were obtained at the U.S. Bureau of Standards. One gramme 
of celluloid was dissolved in 400 grammes of amyl acetate, and a 
drop was placed on the surface of clean water and allowed to 
evaporate. ‘The film was transferred to glass and its thickness was 
found to be 30 Angstrém units (3 x 10-7 cm.). Films obtained 
similarly from a solution of 1 gramme in 800 grammes of amyl 
acetate would just hold together, but films from a solution of 1 
gramme in 1200 grammes of amyl acetate broke up as the amyl 
acetate evaporated. If the thickness of the films decreased in the 
same ratio as the dilution of the solution, the thickness of the films 
just too thin to hold together would be 10 Angstrém units, and this 
agrees with the thickness calculated from the density of celluloid, 
the concentration of the solution, and the area of the film. Hence 
the molecular complex of celluloid is not more than 10 Angstrom 
units (= 10°’ cm.) in diameter.* This agrees with results of similar 
experiments on films of oil and other organic substances. 

From the description of the material as “ celluloid,’ the authors 
probably employed either ordinary celluloid or cinema film in these 
experiments, 2.e., a complex of cellulose nitrate and camphor. The 
final film, however, would probably be cellulose nitrate alone, since 
camphor is sufficiently soluble in water to be completely extracted 
from such a film in a very short time. It is probable that measure- 
ments of the thickness of such films would yield valuable results if 
they were correlated with the viscosity of the nitrocellulose, which is 
believed to be a function of the complexity of its molecular structure. 
The problem of deciding how the film is built up from the various 
sizes of particles believed to be present in these solutions is, however, 
much more difficult. 

Experiments by Laird 3’ throw some doubt on the validity of 
the calculations by which Barton and Hunt checked the thickness 
of the films. She measured the thickness of very thin films by an 
interferometer method, and assuming that the optical thickness is 
the true thickness, found that the density of the celluloid remained 

-approximately constant at 1-41 down to a thickness of 400 uu(= 4 x 
10° cm.). Below this thickness, the density begins to increase. In 
the neighbourhood of 60 uu (6 x 10°%cm.) it is about 2 and at 30 wu 
(3 x 10cm.) it is about 2-5. 

BEG ye 28; 


164 Cellulose Ester Varnishes 


Chromatic Emulsions. 


In experimenting on the production of transparent emulsions, 
by emulsifying two immiscible liquids of equal refractive index, 
Holmes and Cameron °° succeeded in obtaining some very striking 
chromatic emulsions using a solution of cellulose nitrate in amyl 
acetate as the emulsifying agent. Their directions for a lecture 
experiment as are follows: 4 volumes of glycerol and 4 volumes 
of a 2 to 4% solution of cellulose nitrate in amyl acetate are shaken 
together to form an emulsion. To this is added 5—10 volumes of 
benzene (toluene may be substituted), then more glycerol until the 
emulsion is somewhat viscous, and finally benzene again in small 
quantities until the colours appear. The order in which the pairs 
of colours appear with the addition of benzene is yellow and blue, 
pink and green, peacock-blue and pale yellow. If more benzene is 
added, the colours disappear and the emulsion becomes milky, but 
the addition of amyl acetate will bring them back in the reverse 
order. Chromatic emulsions have been known for many years, 
and can be produced, for example by emulsifying oil of turpentine 
and glycerol. Holmes and Cameron’s emulsions, however, are more 
beautiful and permanent. If the emulsions are allowed to stand, 
a cream separates which shows the colours very clearly, and this 
can be separated from the lower layer. A quantity of this emulsion 
made by the writer nearly two years ago has retained its chromatic 
properties without change, except that the cream is now a semi- 
solid gel showing brilliant pink and green interference colours, 
altering with change of temperature. 

The theory of chromatic emulsions is discussed by Holmes and 
Cameron (loc. cit.). They are due to the presence of two phases 
nearly equal in refractive index but differing considerably in optical 
dispersive power. : 


Ne 


Transport of Cellulose Ester Solutions on British Railways.* 


Cellulose ester varnishes are classified by the British railways 
under the general heading of Inflammable Liquids, Class A, and 
the regulations controlling the traffic are printed in the “ General 
Railway Classification of Goods” published by the Railway Clearing 
House. It is possible to find variations in the classification, but 
the policy of the railway companies, after having safeguarded the 
interests of passengers and of the owners of other merchandise in 
transit, is to meet the requests of traders as far as they can. The 

* Mr. J. H. B. Jenkins, Chief Chemist of the London and North-Eastern 


Railway, has kindly read this section and checked the accuracy of the facts ; 
for any comments or opinions the author alone is responsible. 


Precautions Necessary 165 


variations, therefore, arise from the fact that different industries 
have asked for different facilities, which have sometimes been 
granted. The writer is glad to testify to the sympathetic hearing 
which manufacturers are given when they appear before a combined 
meeting of the chief chemists of the British railways. 

The page references in the paragraphs which follow are to the 
current issue of the General Railways Classification of Goods, 
dated Ist April, 1924. 

‘The varnishes appear in the classification under some unfamiliar 
names, derived sometimes from the trade names of the products 
made by the manufacturers who carried on the pioneer negotiations 
with the railway companies. Thus the cellulose nitrate varnishes 
appear as N.E. xylonite solutions (pp. 355 and 356), collodion 
(pp. 357 and 359), xylonite solution (p. 357), nitrocellulose solution 
(pp. 358 and 359), the initials N.E. signifying that the solution 
contains no ether. Solvents supplied for the dilution of the varnishes 
are described as N.E. xylonite thinnings, xylonite thinnings and 
collodion thinners. Aeroplane varnish or dope appears on p. 357, 
no distinction being made between nitrate and acetate dopes in 
view of the fact that the solvents in each constitute a similar fire 
danger. | 

The distinguishing feature of the inflammable liquids in Class A 
is that their flash point is below 73° F., the determination of the 
flash point being carried out in accordance with the directions of 
the first Schedule to the Petroleum Act of 1879. The instructions 
are reproduced on pp. 538-541. 

The regulations may be summarised as follows: Cellulose esters 
under Class A were for a period subdivided (in effect) into those 
which contained ether and those which did not. The high vapour 
pressure of ether was regarded by the railway companies as justi- 
fying an extra precaution, which took the form of limiting the 
capacity of the containers, in which the solution might travel, to 
10 gallons. Solutions not containing ether (N.E.) were allowed to 
travel in approved drums or barrels of anything up to 100 gallons 
capacity. This distinction has now been effaced by the entry on 
p. 359, allowing collodion also to travel in approved drums or barrels 
up to 100 gallons capacity. Since all solutions containing ether are 
usually grouped under the term collodion, the effect of this entry 
is in practice to put solutions which contain ether on the same basis 
as those which do not. Collodion thinners, however, p. 358, are 
still subject to the 10-gallon limit. ‘ 

The specification for drums of 20 to 100 gallons capacity for this 
traffic is given on pages 527-528, and may be summarised thus : 


166 Cellulose Ester Varnishes 


They must be made of best soft iron or mild steel, with joints either 
riveted, soldered or autogenously welded; strengthened at each end 
by a strong iron or steel hoop shrunk on to the body, and provided 
with two rolling hoops welded or shrunk on. An alternative to 
the rolling hoops is the provision of two corrugations in the body 
protected by cover strips to specification. Barrels must have a 
bilge proportional to their length, the 33-inch barrel requiring a 
21-inch bilge; they must also have end hoops, but need not have 
rolling hoops. The minimum thickness of the metal varies according 
to the size of the barrel according to the following table :— 


Capacity. Body. Ends. 
20 galls 14 B.G 16 B.G 
25—35_ ,, i ae 15 ,, 
36—65_ ,, 10 ,, 


it hs 
12 (barrels) 
66—100 ,, 9 5, 10 (drums) 


Each drum or barrel must have a well-fitting screw bung, which 
must not project above the rolling hoops when screwed in. The 
boss must be autogenously welded or riveted and soldered to the 
body or end of the barrel. The drums must stand a hydraulic or 
air test of 10 lb. without leaking; this test to be repeated annually 
and whenever they show signs of damage. They must also be 
repainted or regalvanised as required to protect them from rust. 
The last two provisions are important—an air space of 5% must 
be allowed in filling and the bungs of empty barrels and drums must 
be securely screwed in before they are returned. The provision of 
the air space is to prevent bursting of full drums by rise of tem- 
perature, and the regulation about bunging the empties guards against 
the danger of accidental ignition of inflammable vapours remaining 
in the drum. 

The specification for smaller drums, p. 524, of 1 to 15 gallons 
capacity is on similar lines, the differences being as follows: Streng- 
thening hoops at each end are only required if the drum exceeds 
12 gallons in capacity. The minimum thickness is 20 B.G. The 
screw bung and boss must be faced, and a washer provided, to 
ensure a good joint. (The bungs on these small drums are frequently 
wider than on the larger drums and barrels.) Drums for the export 
trade may have a bung or shive driven into a plain cylindrical neck, 
and covered with a soldered metal cap. In this instance the neck 
must be not more than 1} inches in diameter, and the top must be 
from 4 inch to } inch below the level of the chime. 


Precautions Necessary 167 


The containers smaller than 1 gallon are used chiefly in the 
retail trade, and for a description of these reference must be made 
to the original classification. The chief safeguard consists in packing 
them in cases with sawdust. 

Aeroplane dopes or varnish may travel in drums or taper-neck 
cans of 10 gallons maximum capacity (p. 529), and otherwise in 
wide-mouthed drums and taper-neck cans of 5-gallon and 10-gallon 
capacity (p. 533). 

The thickness of metal is given in the following table :— 


5 gall. — 10 gall. 
NRE A UAE 8 sje os 5 nin d's ve ss'tecvisvcgss 22 B.G. 20 B.G. 
Body, shoulder and neck of cans...... = aa 
es 0 20 B.G. 


BPG COTE OEICAIS 0 ogi ss daceeedeuse os ver. 
The ends of drums and the bottoms of cans must be stamped 
out with a flange of 3 inch to 1 inch, and placed on the body with 
the flange outward. These ends must either be swaged and welded, 
or welded and hooped as directed. 

Two methods of closing the drums are allowed: (1) A cast-iron 
plug not exceeding 24 inches in diameter screwed with a leather 
washer into a flanged mild steel collar welded to the head of the 
drum at one side; (2) a well-fitting cork bung in a smooth neck of 
4 inches diameter, with a metal capsule reeled over the wired edge 
of the neck. 

Drums must be provided with handles welded, or riveted and 
welded, to the heads, but the handles and the screw bungs must 
not project beyond the end of the drums. 

The taper-neck cans must have the shoulders autogenously 
welded to the cylindrical body, and the necks similarly attached 
to the shoulders. A 3-inch steel or brass ring must be welded or 
soldered respectively to the neck after galvanising. The ring is 
screwed to receive a screwed cast-iron plug with a leather washer. 
Two wrought-iron handles are welded on, one on each side of the 
shoulder. 

A method is described for sealing the drums by means of a 
wire passing through the bung and the rim of the drum. Alternative 
arrangements are sanctioned. 

The principal special regulations for the traffic are that each 
package must bear a conspicuous label “‘ Highly Inflammable ”’ ; 
drums must be painted red at each end with the words “ Highly 
Inflammable ”’ in white letters; loading must be performed in day- 


168 Cellulose. Ester Varnishes 


light, and the traffic must not be stored in any of the railway 
companies’ enclosed sheds and warehouses. A special form of 
consignment note is necessary. 

In regard to these regulations in general, it may be said that 
they are consistent with the principle mentioned in an earlier 
chapter of this book, that the danger of cellulose ester solutions is 
a function only of the inflammable solvents which they contain, 
and not of the solid contents. On the surface they appear somewhat 
onerous, but prospective users of such solutions should note that 
whatever burden they may place on the industry is borne almost 
entirely by the supplier and not by the user, except in so far as special 
regulations for the traffic must add something to the cost of the 
commodity. All that the regulations call on the consumer to do 
is to employ the special consignment note when returning empty 
containers to the supplier, and to replace the screw bungs securely. 


REFERENCES. 


_ Precautions necessary in using Cellulose Hster Varnishes.—! Annual 
Report of Chief Inspector of Factories, 1917 [Cd. 9108], p. 18 (Report on 
Doping in Aircraft Works by W. H. Smith, with advice on ventilation). 
2 EK. Hausbrand, ‘‘ Drying by means of Air and Steam ”’ (Scott Greenwood). 
%,. J. Glaisher, Hygrometrical Tables (Taylor and Francis). 

Analysis of Cellulose Ester Varnishes.—* J. R. Lorenz, J. Amer. Leather 
Chem. Assoc., 1919, 14, 548-556. 5 A. D. Conley, J. Ind. Eng. Chem., 1915, 
7, 882. ® H. Dubovitz, Chem. Zeit., 1906, 30, 936-937. 7 W. H. Gibson 
and L. M. Jacobs, Trans. Chem. Soc., 1920, 117, 473. ® E. W. J. Mardles,- 
Trans. Faraday Soc., 1923, 18, 337. 

Solvent Recovery.—® J. I. O. Masson and T. L. McEwan, J. Soc. Chem. 
Ind., 1921, 40, 32-387. 1° J. Brégeat, B.P. 128,640 (1917). 41 Technical 
Records of Explosives Supply No. 8, ‘‘ Solvent Recovery.” 1% P. Drinker, 
J. Ind. Eng. Chem., 1921, 18, 831-835. 

See also C. S. Robinson, ‘‘ Recovery of Volatile Solvents’? (Chemical 
Catalog Co., 1922). M. Deschiens, ‘‘ Procédés industriels de récuperation 
des dissolvants volatils,’’ Rev. des Prod. Chim., 15/5/1920. M. Ponchon, 
Chim. et Ind., 1918, 1, 481-491 (‘‘ Theory of Refrigeration Methods ’’). 
E. C. Worden, ‘‘ Technology of Cellulose Esters,” Vol. I., Pt. IV (List of 
patents). W. D. Milne, Chem. Age (New York), 1923, 31, 201-205 (Deals 
with fire dangers in solvent recovery, with some description of various pro- 
cesses). J. H. Wild, India-Rubber Journal, 1923, 65, 313-322 (primarily an 
account of solvent recovery in the rubber industry). P. Brasseur, Fed 
Ind, Chim. Belg., 1922, 1, 155-167, 315-327. A. Djeinem, ‘‘ Le Caoutchouc et 
La Gutta-Percha,’’ 1919, 16, 9980-9984. H. Carstens, Z. angew. Chem., 1921,. 
34, 389-392 (an account of the Bayer Co.’s process of solvent recovery by 
activated charcoal). E. C. Williams, J. Soc. Chem. Ind., 1924, 48, 97-1127 
(‘‘ Benzol Recovery by Adsorption by Silica-gel ”’). 

Scientific Applications.—'* E. C. Worden, ‘‘ Nitrocellulose Industry,” 
Vol. I., p. 812. 14 G. 8. Walpole, Biochem. J., 1915, 9, 284-308. 15 FP. 
Keeble, Nature, 1924, 114, 56. 1° W. Brown, ibid., 1915, 8, 591-617; 1915, 
9, 320; 1917, 11, 40-57. 17 C. J. Farmer, J. Biol. Chem., 1917, 32, 447-— 
453. 18 A. H. Eggerth, ébid., 1921, 48, 203-221. 1% R. Gans, Ann. Physik, 
1920, [iv], 62, 327-330. #9 J. M. Looney, J. Biol. Chem., 1922, 50, 1-21, 
21 G. Wegelin, Koll.-Z., 1918, 18, 225-239. 2 §. L. Bigelow, J. Amer. 
Chem. Soc., 1907, 29, 1675-1692. 3 J. Duclaux and J. Errera, Rev. gén, 
Collotdes, 1924, 2, 130-139 (see J. Soc. Chem. Ind., 1924, 43, B, 815). % F. E. 


ght aati 


Precautions Necessary 169 


Bartell and D. C. Carpenter, J. Phys. Chem., 1923, 27, 101-116. 25 G. 
Preuner and O. Roder, Z. Elektrochem., 1923, 29, 54-64. 2° J. Duclaux, 
B.P. 203,714/1922. 27 R. Zsigmondy and W. Bachmann, Z. angew. Chem., 
1918, 11, 413; J. pr. Chem., 1918, 108, 119-128. #8 W. Bachmann, Koll.-Z., 
1920, 37, 138; Z. angew. Chem., 1919, 32, 616. °° G. Jander, Z. angew. 
Chem., 1922, 35, 721; <dbid., 1923, 36, 505. °° A. Kling and A. Lassieur, 
Chim. et Ind., 1923, 10, 42. °! H. Bechhold and L. Gutlohn, Z. angew. 
Chem., 1924, 37, 494-497. 

See also C. F. Tinker, Trans. Faraday Soc., 1917, 37, 133-140. Wo. 
Ostwald, ibid., 1921, 16, 89-93. 

Miscellaneous Scientific Uses.—** R. W. Wood, Physical Optics (1911), 
p. 97. 33 Idem, ibid. (1914), p. 172. 34 Lord Rayleigh, Phil. Mag., 1917, 
34, 423-428. %5 C. V. Boys, “‘ Soap Bubbles ”’ (S.P.C.K.), 117-120. °° V. P. 
Barton and F. L. Hunt; ’Noture, 1924,,114, 861.°:%7 Elizabeth, R. Laird, 
Physical Review, 1922, 19; &84.; 738 °H:.N. Beimes ‘artd?)/ H.:Camieron, :/. 
Amer. Chem. Soc., 1922, 44, *73574, 2 233 230235 7% 0553 7252 2 8 3 ae 8 Sao 


» 28 ra ,990 ©>. 9 ae 
) eo % "3 5 ) 1 9 
oe 





INDEX OF NAMES 


ABEL, F., 33, 44 

Archbutt, oe ap Deeley, R. M., 112 

Archer, F, = 

Aston, FE, W., pa 

Ayres, A. E., 119 

Bachmann, W., 162. See also Zsig- 
mondy. 

Baker, F., 17, 69, 73 

Balls, W. L., 26, 66. 

Bancroft, W. D., 17 

Barnett, W. L., 55, 67 

Barr, G., 56 

Barr, G., and Bircumshaw, L. L., 55, 77 

Bartell, F. E., and Carpenter, D. C., 161 

Barthélemy, i. 45, 55 

Barton, V. P., and Hunt, F. L., 163 

Bayer, F. & Co., 20, 48, 50 et seq. 

_Bechhold, H., and Gutlohn, L., 162 

Becker, E. See Schwalbe, C. G. 

Bell, M. See Hake, C. N. 

Berg, van den, J. C. See Boeseken, J. 

Bergmann, E., and Junk, A., 44 

Berry, A. J. See Fenton, H. J. H. 

B.E.8.A. See British Engineering 
Standards Association. 

Bevan, E. J. See Cross, C. F. 

Bigelow, J. See Maynard, F. 

Bigelow, S. L., 161 

Bingham, E. C., 59, 60, 79, 92 

Bingham, FE. fi, and Hyden, W. S., 80 

Bireumshaw, L. L. See Barr, G. 

Birtwell, C., Clibbens, D. A., and Ridge, 
B. P., 45 

Boeseken, J., van den Berg, J. C., and 
Kerstjens, A. H., 27, 66, 91 

Bolsing, Fr. See Verley, A. 

Bottger, R., 14 

Boys, C. V., 162 

Brasseur, P., 168 

Brégeat, J., 158 

Bregenzer, A. See Knoevenagel, E. 

Breguet, A. See Meunier, L. 

Briggs, J. F., 48 

British Engineering Standards Associa- 
tion, vii, 43, 44, 45, 53, 55, 56, 60, 99, 
101, 102, 104, 106, 107, 109, 110, 
111, 112, 122, 155 

British Pharmacopeceia, 141 

Brown, W., 161 

Bruin, de, G., 45 


Caille, A., 23 

Cameron, D. H. Sac Holmes, H. N. 

Carpenter, D.C. See Bartell, F. E. 

Carstens, H., 168 

Cellonite Co., 48 

Celluloid Co., 18 

Clément, L., and Riviére, C., 23, 99, 119, 
120, 128, 137, 139 

Clibbens, D. A., and Geake, A., 45 

Clibbens, D. A. See also Birtwell. 

Condé, G. E., 23, 134, 142 

Conley, A. D., 153 

Crane Chemical Co., 18 

Crane, F., 17 


Crockett, H. G., 136 

Cross, C. F., 26, 27, 57 

Cross and Bevan, 19, 29 

Cross and Dorée, 29 

Crum, W., 15 

Custom and Excise, Board of, 42 


De Waele, A., 81 

Deeley, R. M. See Archbutt, L., 

Denham, W. S., and Woodhouse, H., 25 

Departmental Committee on Heat Test, 
44 

Deschiens, M., 136, 168 

Djeinem, A., 168 

Doerflinger, W. F., 108 

Domonte, F., and Menard, 15 

Donnan, F. G., 91 

Dorée, C., 26. See also Cross, C. F. 

Dreyfus, Bros., 48 

Dreyfus, H., 20, 48, 51, 52 et seq. 

Drinker, P., 23, 48, 121, 158 

Dubovitz, H., 154 

Duclaux, J., 162 

Duclaux, J., and Errera, J., 161 

Duclaux, J., and Wollmann, E., 27, 66, 
Tl, 72, 91 


Eberstadt, O., 54. 

venagel, E. 
Eggerth, A. H., 161 
Eichengrin, A., 20 
Entat, M., and Vulquin, E., 55 
Errera, J. See Duclaux, J 
Esselen, G. J., Jnr., 88, 114 
Evans, H. G., 115 


See also Knoe- 


Factories, Chief Inspector of, 115, 143 

Farmer, C. J., 161 

Fenton, H. J. H., and Berry, A. J., 
54, 88 

Field, W. D., 18 

Finkener, 112 

Fischer, E., 23 

Flaherty, E. M., 133, 134 

Franchimont, A., 19 


Gans, R., 161 

Gardner, H. A., 130 

Gardner, H. A., and Parkes, H. C., 97 

Geake, A. See Clibbens, D. A. 

Gibson, W. H., and Jacobs, S. M., 60, 155 

Gibson, W. H., and McCall, R., 74, 75 

Gibson, W. H., Spencer, L., and McCall 
R., 36 

Girard, A., 19 

Gladstone, J. H., 15 

Gmelin, L., 14 

Goldsmith, J. N., 83 

Graham, T., 58 

Green, A. G., 24 

Gutlohn, L. See Bechhold, H., 


Hadow, E., 15 

Hake, C. N,, and Bell, M., 45 
Hall, A. J., 24 

Harrison, W., 26, 27 


171 


172 


Hartig, 15 

Hatschek, E., 83 

Hatschek, E., and Humphry, R. H., 
83 

Hausbrand, E., 168 

Haworth, W. N., and Hirst, E. L., 25 

Haworth, W.N., and Leitch, G. C., 25 

Herrmann, K. L., and Radel, F. J., 137 

Herzog, R. O., and Jancke, W., 27 

Higgins, C. A. See Small, J. O. 

Highfield, A., 74 

Hirst, E. ‘ See Irvine, J. C., and 
Haworth, W.N. 

Hogrefe, J. See Knoevenagel, E. 

Holmes, H. N., and Cameron, D. H., 164 

Humphry, R. H. See Hatschek, E. 

Hunt, F. L. See Barton, V. P. 

Hunt, R., 15 

Hutchison, A. M., 15 

Hyden, W.8. Sce Bingham, BE. C. 


Irvine, J. C., and Hirst, E. L., 25, 26, 
27, 67 


Jacobs, 8. M. See Gibson, W. H. 
Jancke, W. See Herzog, R. O. 
Jander, G., 162 

Joyner, R. ‘fh. 45 

Junk, A. See Bergmann, E. 


Kerstjens, A. H. See Béeseken, J. 

Kirkpatrick, S. D., 40, 118, 134 

Kling, A., and Lassieur, A., 162 

Knoevenagel, E., 84, 91, 161 

Knoevenagel, E., and Bregenzer, A 5 92 

Knoevenagel, E., and Eberstadt, O., 54, 
92 

Knoevenagel, E., Hogrefe, J., and 
Mertens, F.., 92 

Knoevenagel, E,, and Motz, R., 92 

Knoevenagel, E., and Volz, E., 92 

Knop, W., 14 

Kugelmass, I. N., 76 


Laird, E., 163 

Lassieur, A. See Kling, A. 
Laue, von, 27 

Lederer, L., 19, 52 

Legge, T. "See Willcox, W. H. 
Leitch, G. C. See Haworth, W. N. 
Lewkowitsch, J., 112 
Leysieffer, a. 29 

Looney, J. M., 161 

Lorenz, J. R., 151 

Lucas, A., 138 


MacDonald, G. W., 14 

MacNab, W., 45 

Mardles, E. W. J., 62, 78, 82, 87, 88, 91 

Marshall, A., 45, 101 

Masson, J. I. O., 75 

Masson, J. I. O., and McCall, R., 58, 72 

Masson, J. I. O., and McEwan, T. L., 
158 

Maxwell, C., 59 

Maynard, Jui and Bigelow, S., 15 


Index of Names 


McCall, R. See Gibson, W. H., and 
Masson, J.I. O. 

Menard. See Domonte, F. 

Mertens, F. See Knoevenagel, E. 

Meunier, L., and Breguet, A., 83 

Miles, G. W.., 20, 47, 50, 51 

Milne, W. D.; 168 

Ministry of Munitions, Technical Records 
of Explosives Supply, 31, 158 

Mork, H. §., 52 

Morrell, R. S., 115 

Mosenthal, de, H., 68 

Motz, R. See Knoevenagel, E. 

Mougey, H. C., 132 


Napper, S., 27 
Nathan, F., 37 
Nobel, A., 18 


Ost, H., 51 et seq., 54 
Ostwald, W., 43, 60 
Ostwald, Wo., 169 
Otto, F., 14 


Parkes, A., 15, 16, 18 

Parkes, H.C. See Gardner, H. A. 

Parry, E. J., 115 

Pellen, M., 16 

Pelouze, J., 14, 15 

Perkin, W. H., Jnr., 22 

Pierce, F. T., 26 

Piest, C., 45 

Piest, C., Stich, E., and Vieweg, W., 
35 


Poiseuille, 60 

Ponchon, M., 168 

Porter, A. W., 83 

Preuner, G., and Roder, O., 162 
Punter, R. A., 34 


Radel, F. J. See Herrmann, K, L. 
Ramsbottom, J. E., 23, 120 
Rassow, B., 29 

Rhéne. See Usines du Rhéne. 
Ridge, B. P. See Birtwell, C. 
Riviere, C. See Clément, L. 
Robertson, R., 23, 41 

Robertson, R., and Smart, B. J., 44 
Robinson, C. S., 168 

Roder, O. See Preuner, G. 


Saillard, B., 17 

Sapojenikov, M. A. W., 45 
Schonbein, OC. F., 14, 15 

Schrimpf, A. See Schwalbe, C. G. 
Schultze-Tiemann, 44 
Schiipphaus, R., 17 
Schiitzenberger, P., 19 


. Schwalbe, C. G., 23 


Schwalbe, C. G., and Becker, E., 26 
Schwalbe, 0. G., and Schrimpf, Ag 29 
Schwarz, H., 35, 70 

Segundo, de, E. P., 45 

Sheppard, S. E., 60 

Small, J. O., and Higgins, C. A., 45. 
Smart, B. a; See Robertson, R. 


Index of Names 173 


Smith, W. H., 115, 168 
Spencer, L. See Gibson, W. H. 
Spiers, C. H., 110 

Spill, D., 16 


Spilsbury, B. See Willcox, W. H. 


Stevens, J. H., 17, 18 
Stich, E. See Piest, C. 


Thornley, T., 45 

Tinker, C. F., 169 

Tucker, W. K., 135 
Tunison, B. R., 101, 105, 107 


Ullmann, F., 23 
United States Pharmacopoeia, 141 


Usines du Rhéne, 20, 48, 49 et seq. 


Verley, A., and Bolsing, Fr., 110 
Vieweg, W. See Piest, C. 
Visser, C., 71 — 


Volz, KE. See Knoevenagel, E. 
Vulquin, E. See Entat, M. 


Waele, de, A., 81 

Walpole, G. 8., 159, 160 

Wegelin, B., 161 

Wiesel, J. C., 131 

Wild, J. H., 168 

Willcox, W. H., 115 

be ee W.H., Spilsbury, B., and Legge, 

-» 115 

Williams, E. C., 168 

Wollmann, E. See Duclaux, J. 

Wood, R. W., 162 

Woodhouse, H. See Denham, W. 8S. 

Worden, E. C., vii, 23, 45, 49, 50, 52, 
68, 90, 115, 159, 168 


Zsigmondy, R., and Bachmann, W., 169 
Zihl, E., Lil 


INDEX OF SUBJECTS 


ACCUMULATOR cases, 22 
Acetic acid, 15, 16 
Acetic anhydride, 19, 46 
Acetins, 17 
Acetoethyl-o-toluidide, 69 
Acetone, 15, 18, 21, 27, 37, 69, 71 
British Government Specification, 100 
manufacture, 101 
viscosity determination in, 29 
Acetone oil, 17 
specification of, 107 
Acetylene tetrachloride. 
chloroethane. 
Acetylation process, 48 et seq. 
plant for, 53 
Acid balance, 31 
Acid ratio in nitration, 32 
Acid revivification, 31 
Aeroplane dopes, 16, 20, 48, 94 
allied practice, 121, 127 et seq. 
adhesion test, 123 
British practice, 122 
brittleness, 123 
doping schemes, 122 
elasticity, 123 
formule, discussion of, 124 et seq. 
German practice, 127 
inflammability, 122, 123 
manufacture, 119 et seq. 
for night bombers, 128 
protective, 128 
rate of burning, determination, 123 
softness, 121 
tautening test, 123 
transport, 165 et seq. 
Aeroplane fabrics 
action of sunlight on, 128 et seq. 
enumeration of, 121 
specific strength of, 120 
strength after doping, 120 et seq. 
Air, moisture in, 144 et seq. 
Alabaster, preservation of, 139 
Alcohol. See Ethyl alcohol. 
Alum, 16 
Aluminium mediums, 132 et seq. 
Aluminium paints, 138 
Amber, preservation of, 194 
Ammonium phosphate, 16 
Amy] acetate, 17, 18, 69 
determination in varnish, 161 
properties, 17, 106 
specification, 107 
Amy] alcohol, 17, 18 
specification for, 110 
Aniline, 16 
Antiques, preservation of, 138 et seq. 


See Tetra- 


Barrels, 166 
approved for transport, 166 
labelling, 167 
Bathroom fittings, 138 
Benzene, 17 
as diluent in acetylation, 52 
specification for, 109 
Benzine. See Petroleum spirit. 
Benzol. See Benzene. 


174 


Benzyl alcohol, specification, 111 
Blooming, 144 et seq. 
prevention of, 148 et seq. 
Blushing. See Blooming. 
Bronze mediums, 16, 132 et seq. 
paints, 138 
powder, action of, on nitrocellulose, 


Brushing lacquers, 129 

Butyl acetate, 18 
specification for, 106 

Butyl alcohol, 18, 21 
manufacture of, 101 
specification for, 110 


Cadmium iodide, 16 
Calcium oxalate, 16 
Camphor, 16 
and cellulose nitrate, 15 
in varnishes, significance of, 152 
Camphor-alcohol solvent, 37 
Carbon tetrachloride, 52 
Carbonyl group and solvent power, 69 
Cardboard varnishes, 138 
Castor oil, in varnishes, 130 
specification for, 112 
Cellite L., 20 
Cellobiose, 25, 27 
Celloidin, 439 
Celluloid, cements, 22 
gilding, 140 
Cement floors, primers, 139 
Cellulose, 24 et seq. 
acetolysis of, 25 
acetylation of, 19 et seq. 
constitution of, 25 et seq. 
constitution of, Rontgen ray, 27 
depolymerisation of, 28 
formula of, 24-26 
formula, polymerisation of, 26 
hydrolysis of, 25 
methylation of, 25 
molecule, 27 
molecule, dimensions of, 28 
mucilage, 27 
nitration of, 14, 30 et seq. 
nitration of, acid bath, 32 
nitration of, conditions affecting solu- 
bility, 32 
nitration of, control of, 33 
nitration, processes of. See Nitration 
processes. 
nitration of, temperature of, 33 
nitration of, time of, 33 
oxidation of, 25 
reactions of, 24 et seq. 
triacetate, 25 
trinitrate, 25 
varieties of, 13 
varnishes. See Cellulose. ester var- 
nishes. 
viscosity of, 28 
viscosity, and growth of, 65 
viscosity of, specification, 36 
Cellulose acetate (s), acetyl content, 
_ determination, 54 


Index of Subjects 175 


Cellulose acetate (s), (conid.)— 

acetyl content, theoretical, 64 

analysis, 54 

benzyl alcohol and cyclohexanone, 78 

bromophenylhydrazones, 67 

charring test, 55 

classification (Ost), 51 

dopes. See Aeroplane dopes. 

history, 19 et seq. 

hydration of, 20 

hydroxyl content of, theoretical, 64 

inflammability of, 93 

manufacture of, in Europe, 21, 48 

phenylhydrazones, 67 

properties of, 54 

solubility of, 20 

solubility of, in aqueous acetone, 51 

solubility of, modifications of, 51, 
52 et seq. 

solubility of, test of, 56 

solutions, in acetic acid and water, 78 

solutions in aniline and acetic acid, 78 

solutions, fractional precipitation of, 66 

solutions of, inmixed solvents, 77 et seq. 

solutions of, Tyndall effect in, 81 

solutions of, viscosity of, 48, 77 et seq. 

solutions of, viscosity, acetone and 
water, 77 

solutions of, viscosity, acetone and 
benzene, 77 

solutions of, viscosity, acetone and 
alcohol, 77 | 

solvents of, 68 

solvents of, classification of, 99 

specification of, 54 et seq. 

stability tests of, 55 et seq. 

sulphur determination in, 55 

swelling of, 84 et seq. 

swelling of, equilibrium, 87 

swelling of, heat change, 87 

swelling of, surface tension relations, 
85 

swelling of, viscosity relations, 85 

swollen, dyeing of, 84 

swollen, saponification of, 84 

swollen, and solvent, distribution of 
components, 84 et seq. 

varnishes. See Aeroplane dopes. 

varnishes, analysis of, 155 et seq. 

varnishes, for artificial leather, 136 

viscosity test, 56 

viscosity test, solvents for, 56 

volume change on dispersion, 87 

Cellulose esters (not specified). 

association with solvent, 88 et seq. 

constitution of, 63 et seq. 

dissolution, phenomena accompanying, 
58 


hydrolysis during formation, 65 
particles, dimensions of, 91 
variations in, 96 


Cellulose ester solutions. See also 


Cellulose ester varnishes. 
constitution of, 88 et seq. 
inflammability of, 93 
nomenclature of, 14 


Cellulose ester solutions (contd. )— 


properties of, 13 et seq. 

viscosity of, 21, 28 

viscosity and concentration, 69 et seq. 
viscosity and temperature, 63 
viscosity, significance of, 28 


Cellulose ester varnishes, 


absorption by silver, 139 

analysis of, 149 et seq. 

application, methods of, 128 et seq. 

applications of, 119 et seq. 

brushing, 128 

centrifugalisation of, 118 

clarification of, 118 et seq. 

dipping, 129 

filtration of, 118 

flash point of, 165 

flowing, 130 

ingredients, measurement of, 116 

manufacture of, 116 et seq. 

manufacture of, time required in, 117 

moulding, 130 

settling, 118 

spraying, 129 

transport of, on British Railways, 
164 et seq. 


Cellulose nitrate (s), 


and alcohol at low temperature, 76 
blending of, 41 

boiling, 41 

dehydration of, centrifugal, 42 
dehydration of, Dupont press, 42 
determination of, 151 et seq. 
disintegration of, by acetone and water, 


drying, 42 

and ether, at low temperatures, 76 

explosive, 15 

films, thin, density of, 163 

films, thinnest, dimensions of, 163 

‘fuming off,” 33 

heat tests of, 43 et seq. 

history of, 14 et seq. 

hydroxyl content of, theoretical, 64 

inflammability of, 36, 93 

manufacture of. See Nitration pro- 
cesses. 

nitrogen percentage, 15, 36 

nitrogen percentage, determination of, 
of 


nitrogen percentage, determination of, 
in lacquers, 150 

nitrogen percentage, theoretical, 64 

nomenclature of, 37 

poaching, 41 

precipitation of, by aqueous electro- 
lytes, 154 

properties of, 42 et seq. 

solubility of, 15, 37, 42 

solutions. See also Cellulose nitrate 
varnishes. 

solutions, fractional precipitation of, 
27, 66 

solutions, viscosity of, 27 

solutions, viscosity of, acetone and 
water, 72 et seq. 


176 


Cellulose nitrate (contd.)— 
solutions, viscosity and concentration 
of, 69 et seq. 
solutions, viscosity, determination of ,43 
solutions, viscosity, ether-alcohol, 14 
et seq. 
solutions, viscosity low, 22, 133 et seq. 
solvents, 68 et seq., 98 
solvents, mixed, 72 et seq. 
‘specification of, 42 et seq. 
stability of, 43 et seq. 
sulphur determination in, 44 
varnishes, 16 
varnishes, analysis of, 149 et seq. 
viscosity, determination of, 43 
washing, 41 
Chalkiness, 144 
Charcoal, activated, 159 
Chloroform, 19 
use in analysis of varnishes, 153 
Chromatic emulsions, 164 
Cinema films, non-inflammable, 47 
Cleanliness of work, 149 
Collodion, 15, 16, 140 
cellulose acetate, 19 
contractile, 141 
films, thin, colours of, 162 et seq. 
films, thin, density of, 163 
films, thin, dimensions of, 163 
membranes, 159 et seq. 
membranes, alcohol index, 161 
membranes, osmosis, anomalous, 161 
membranes, permeability of, 160 et seq. 
membranes, properties of, 160 et seq. 
membranes, standardisation of, 160 
photographic, 15 
surgical, 15 
surgical, formule of, 141 
Collodion cotton. See Cellulose nitrate. 
Collodium, aceto-zthericum, 141 
flexile, 141 
Colloids, 58 
solubility of, 61 
Colza oil, 135 
Cop bottoms, 34 
Copal, 18, 113 
Cordite, 21, 37, 42 
output, relation to cotton treatment, 34 
recovery of solvents from, 158 
Cork, compressed, 140 
Cotton cellulose, action of alkali on, 34, 


67 

alkali soluble, 36 
ash, 35 
copper number of, 36 
cultivation of, 24 
ether extract, 35 
growth of, 27 
methylation of, 25, 67 
moisture in, 35 
purification of, 33 et seq. 
sources of, 24, 34 
specification, for acetylation, 53 
specification, for nitration, 35 
staining test for, 36 

Cotton fibre. See Cotton hair. 


Index of Subjects 


Cotton hair, appearance in 
celluloid, 65 
growth rings, 26 
plasticity, 26 
rigidity, 26 
Cotton-seed oil, 135 
Cotton tissue, nitration of, 34 
Cotton waste, 34 
Crépe stiffening, 139 
Cresol, for solvent recovery, 158 | 
Cuprammonium solvent, 27, 36 


Dammar, 114 
Daub, 136 
Dew point, 146 
determination of, 146 
Dextrose, 25, 28 
Diacetone alcohol, 108 
Dispersion, definition of, 60 
Dissolution, definition of, 61 
Distribution Law, 84 et seq. 
Drums, approved, for railway transport, 
165 et seq. 
approved, labelling, 167 
Drying air, 148 et seq. 


Emulsions, chromatic, preparation of, 
164 
Enamels, 14, 128, 131 et seq. 
Ether, 15, 21 
dispersion of nitrocellulose in, 76 . 
inflammability of, 105 
solutions containing, 
165 
specification of, 105 
Ether-alcohol, 15, 16, 17, 21, 34, 37 
complex, 72 et seq. 
recovery of, 157 et seq. 
solvent power of, 90 
viscosity, 73 
Ethyl acetate, 15, 69 
specification of, 105 
Ethyl alcohol, 14, 17, 21 
solvent power for cellulose nitrate, 76 
specification of, 103 
Ethyl] alcohol-benzene, solvent power of, 
88 et seq. 
alcohol-chloroform, solvent power of, 
88 et seq. 
Ethyl butyrate, 69 
formate, 69, 71 
lactate, specification of, 107 
phthalate, 69 
Ethyl-o-tolyl-ethyl carbamate, 69 
Evaporation, of solvents, 97 Lead 
losses due to, 149 
Explosives, 21 
Explosives Acts, 41 


transport of, 


Faience, protection of, 139 
Fans, electric, 143. 
Faults, recognition of, 144 et seq. 
prevention of, 144 et seq. 
Felt, 16 ; 
Films, non-inflammable, 47. 
thin, properties of, 162 - 


Index of Subjects 


Fire dangers, 143, 148 
Flash point, 165 
Fluidity, 22, 28 
and Tyndall effect, 92 
Fractional precipitation, of solutions, 27, 
66 


French polishing, 137 
Friction, internal, 59 
Fusel oil, 17 

in analysis of varnishes, 151 


Gas masks, 22 

Glass, antique, preservation of, 139 
decoration of, 140 

Glucose. See Dextrose. 

Glycerol, as viscosity standard, 56 

Glycogen, 26 

Gold leaf imitation, 140 

Guncotton, 37 


Hats, stiffening, 140 

Henry’s Law, 86 

Hospital fittings, 138 

Hull fibre, 34 

Humidity, atmospheric, 16, 94 
atmospheric, controlled, 127 
atmospheric, definition of, 145 
atmospheric, measurement of, 146 
atmospheric, table of, 147 

Hydrocellulose, acetylation of, 19 

Hygrometer, 146 


Incandescent mantles, 
140 

Incombustibility, definition of, 121 

Inlaid work, protection of, 139 

Inulin, 26 

Ivory, protection of, 139 


strengthening, 


Kid, patent. See Leather, patent. 


Laboratory results, application of, 96 et 


seq. 
Lacquers, 14, 130 
coloured, 131 
formule of, 130 et seq. 
resin, formule of, 114 et seq. 
Leather, coatings, 16, 136 
dressing, 136 et seq. 
imitation, 135 et seq. 
imitation, fabric for, 135 
imitation, from cellulose acetate, 136 
imitation, viscosity of varnish, 135 
patent, manufacture of, 136 et seq. 
upholstery, manufacture of, 137 et seq. 
Linseed oil, 135 
Linters, 34 — 
nitration of, 40 


Magnesium phosphate, 16 

Mannol, 69 

Mastic, 18, 113 

Matt surface, 132 

Mercuric iodide, 16 

Metals, coating of, 128 et seq. 

Metal lacquers, classification of, 130 


I2 


177 


Metal powder mediums, 130 

Methyl acetate, 69, 71 

Methyl acetone, specification for, 102 

Methyl alcohol. See also Wood spirit. 
determination of, in varnish, 151 
specification for, 104 

Methylene sulphate, 49 

ions pe ethyl ketone, specification for, 

] 

Mixed acid, 14, 15, 31 

Mixers for varnish manufacture, 116 

Motor-car enamels, 132 et seq. 
application of, 134 
failure of, 135 - 
properties of, 133 

Motor-car oil varnishes, 132 

Motor-car stoving enamels, 133 


Nitration processes, 37 et seq. 
American method, 40 
comparison of, 39 
centrifugal, 38 
direct dipping, 37 
displacement, 38 
requirements of, 37 

Nitric acid, specification for, 30 

Nitro-benzene, 16 

Nitro-cotton, 37. 

nitrate. 

Non-inflammability, definition of, 121 


See also Cellulose 


Optical instruments, 138 
Oxycellulose, 25 


Paper varnishes, 138 
Patent leather. See Leather, patent. 
Petroleum Act (1879), 165 
Petroleum spirit, 109 
determination of, 153 
Phenol, solvent power of, 89 
Phenyl-ethyl urethane, 69 
Photographic paper varnishes, 138 
Picture frames, 138 
Pigments, determination of, in varnishes, 
151 et seq. 
incorporation of, 118 
settling of, 149 
Pitting, 144 
Plaster, 16 
Plastic flow, 80 
Polymerised coatings, 13 
Posters, protection of, 138 
Pottery, protection of, 139 
Precautions in using varnishes, 93, 143 
Propyl acetate, 69 
Pyroxylin, 15, 16, 37. 
nitrate. 
Railway Classification, 164 et seq. 
Railway, transport of varnishes on, 
164 et seq. 
Ramie cellulose, 27 
Refractive index and Tyndall effect, 82, 
83 


See also Cellulose 


Resins, 16, 18,-112 et seq., 130 
determination of, 151 et seq. 
Resin lacquers, formule of, 114 


178 


Resin solutions, blending with nitro- 
cellulose, 114 

Rontgen rays, and cellulose structure, 
27 

Rubber, artificial, 18 


Sandarac, 114 

Sericose, 20 

Settling, 149 

Shellac, 18, 112 

Silica, 138 

Silica gel in solvent recovery, 159 

Silver, collodion films on, 162 
absorption of solutions by, 139 
protection of, 139 

Sliver, 34 

Snakeskin, imitation of, 139 

Solids, total, determination of, 150 

Sols, 58 

Solvent power, 21 
definition of, 62 
temperature relation, 63, 74, 76 
use in solvent specification, 102 
viscosity relation, 62 

Solvents (not specified), 18 
anhydrous, 16 
blending of, 94 et seq. 
choice of, 94 et seq. 
comparison of, 61 
complexes in mixed, 73 et seq. 
constitution of, 68 et seq. 
dielectric capacity of, 88 
dilution of, 61 
economic considerations, 18 
evaporation of, 97 
history of, 17 et seq. 
identification of, 154 
inflammability of, 93 
mixed, 72 et seq. 
mixed, complexes in, 73 et seq. 
mixed, de-association in, 79 et seq. 
mixed, dispersion in, 88 et seq. 
mixed, in varnishes, 95 
odour of, 149 
recovery of, 157 et seq. 
specifications of, general clauses, 100 
swelling power of, 84 et seq. 
vapour pressure of, 97 

Spraying, 129 
pressure used in, 134 

Starch, 18, 26 

Stone, protection of, 139 

Sulphuric acid, for acetylation, 19, 46 
for nitration, 30 
specification of, 30 

Sunlight, action of, on fabric, 20 

Surgery fittings, 138 

Swelling in solvents, 84 et seq. 
chemical nature of, 87 


Tale, 16 

Temperature and solvent power, 63 

Tetrachloroethane, specification of, 108 
toxicity of, 108 


Index of Subjects 


Thermometers, wet and dry bulb, 146 
Thinners, 129 
Thinnings, 129 | 
Thread, protection of, 16 
Time-tables, protection of, 138 
Tinsel, protection of, 16 
Toys, decoration of, 16 
Transport of cellulose ester varnishes, 
164 
Triacetin, specification for, 110 
Trimethyl] cellulose, 26 
Trioxy methylene, 49 
Triphenyl phosphate, history of, 11] 
as rétarder of combustion, 122 
specification for, 111 
Toluene, specification for, 109 
Tung oil, in lacquers, 130 
Turpentine oil, 16 
Tyndall effect, 81, 91 
and fluidity, 92 
and yield value, 91 


Ventilation, 93, 108, 143 
in doping schemes, 123 
plenum and vacuum, 144 
Viscose reaction, 90 
Viscometer, falling sphere, 43, 60 — 
Ostwald, 43, 60 
Viscosity, 28, 59, 131, 135 
and concentration, equation connect- 
ing, 69 et seq., 85 
and fractional precipitation, 66 
and solvent power, 62 
and swelling power, 85 
and time of esterification, 65 
definition of, 59 
measurement of, 60 
measurement of, in analysis, 155 
measurements, validity of, 81 
minima in mixed solvents, 72 et seq. 
Volume change on dispersion, 87 


Westron. See Tetrachloroethane. 
Wood, antiques, protection of, 139 
coatings, 16, 138 et seq. 
distillation of, 21 
fillers, 138 
spirit, 15, 16. 
alcohol. 
spirit, specification for, 104 
Wood cellulose, 24, 27, 33 
nitration of, 34 
viscosity and growth, 65 
Worsted, protection of, 16 


See also Methyl 


X-Rays. See Rontgen rays. 

Xyloidine, 37 

Xylonite, solutions, 165 
thinnings, 165 


Yield value, 80 
and Tyndall effect, 92 


Zinc chloride, 19 


a ae 
a ee ee 





D.VAN NOSTRAND COMPANY | 





are prepared to supply, either from 
their complete stock or at 
short notice, 


Any Technical or 
Scientific Book 


In addition to publishing a very large 
and varied number of SCIENTIFIC AND 
ENGINEERING Books, D. Van Nostrand 
Company have on hand the largest 
assortment in the United States of such 
books issued by American and foreign 
publishers. 


All inquiries are cheerfully and care- 
fully answered and complete catalogs 
sent free on request. 





8 WARREN STREET - - 5 New YorxkK 





roxton, Foster. 


: GETTY CENTER LIBRARY CONS 
~g $77 1925a BKS 
etdies ester mama a 


3 3125 00192 3099 
















































































fee ena 


— ge ee ae ae, Pn ete 
a EA Le NO TNT yO ee tt eee ee es 





rl a ibn, Ui ne mE tHE Et et Se et - 
° Bp ne, ~- 


“ Cm : 6 Pas 
sae ae = ~ ~ - ~ nts ni «gat a ag 


q “a0 

ee ee ES IE eed 
I te NI etme 
ere 


ve . 7 ae’ 
Ee ag ee 


Leey es 


ne eeetcn OF, 


We 


m aren = and z 
eo 
SS Ng lar 


WERE WHI DAL 


ON a 


PRR RAMA le a 


So 

Pete Ot Em 

e ~ _ . se te 

ee mal ai ¥ wien : F ~ ; ~ < oe ne ~ heen os ~ aie es * 

eas yt genre - we pC ona! int AE wars mera cred gg i, onal = oe PD tir Ae Mg OO eh ers = Orgter nes 
Pad - Me we vs 
ota 


hd 


NRL TERN We ttee tag Cot baw UR Mirns! 
PTAA CN ARCA RAL RL Ould a ‘ 


AEs BA 
Dares 


i 
7 


pee ie 4 


re Fort oe OIE ig 


a 


Bo ein gf nae ah 
~— mene ee, 


oe Pane 


PRECRLOCUNA GLE RR AER CR IO GE Os EMEA UE OU AOR OEEO RAE S NS ZRPAS AEA RNAGI AEA OMA PCRS 


- WDE NA LID LRA TR WEES Fete Be 


— Poe 


ta 


PAO ER EA Cee 


Hanes 


pee > 


{ 


Hat” OS fra pals 
Hip ea nl te cee A ETN 


, 


ty 

4 \ ue 

ae if 

we Dale hr aveblesianwm sawn bane i8ie 
Rn Y eee i 8 . 4 J 


NUR GRIMES A ERAS AN BLU CEA TOKURA GURL CLAY CTE 


LY e ape me, %, mie 
, a eee) a p - 
Se — - Sl an Stn is 
“ ei ae "A eit SPE Chek SP Le Mn gues ae - 


ns i ASSO Nt ce Et OE Bi Pant eA NE AEE ci ee cnet hi ale 


o 
rT Ps "’ 


